Image compression (Pro)

For each image you compress, Caching Out:

• optimises the original and keeps it at the same URL, so every browser gets the smaller file, Safari and iPhones included. The format and the pixel dimensions do not change, so your pages, srcset and the Media Library need no edit;
• makes WebP and AVIF copies of a JPEG or PNG image for browsers that accept them. They are served by WebP and AVIF delivery, so switch those formats on in Media for the copies to reach visitors. AVIF is offered only when the server can write it. A WebP image needs no copies: the smaller WebP itself is what every browser gets;
• keeps an exact backup of every file it replaces. Restore puts the original back byte for byte and deletes the copies.

Every WordPress size of the image (thumbnail, medium, large and so on) is compressed together with the full-size file.

How much an image shrinks depends on the image: a large photo exported at high quality typically shrinks a lot, an image that was already optimised very little. Results vary by image, and you see the result before anything changes.

Where to find it

• Caching Out > Image compression – a tab in the Optimization group (also in the WordPress sidebar under Caching Out). It lists the images waiting for you under Selected for compression, and below them every compressed image with its saving and Restore, plus Restore all.
• Insights > Page Audit. When the audit reports "Hero image weighs …" for an image in your Media Library, the finding is listed with the fixes and carries Compress this image. Click it to add the image to the Image compression tab; click again to take it off. You can pick several images while you read the audit, then review them one at a time. When an image cannot be compressed, the finding says why instead.
• The Media Library. In list view, an Image compression column shows each image's state: its saving and the files done, Not compressed with a Compress link, or why it cannot be compressed. The Compressed by Caching Out filter above the list shows only the images Caching Out has compressed. A Compress link (or Restore, with the saving) also sits under each image's name and in the attachment details.

Compressing works once your licence has switched Pro on. Restore works without a licence, so a backup is never stranded.

Compressing an image

1. On the Image compression tab, click Review and compress next to an image (or open it from the Media Library).
2. Caching Out works out the smallest file it can make without visible loss and shows it next to the current one, with both sizes. Nothing is written yet.
3. Click either picture for a full-size view with a before / after slider: drag the handle (or use the arrow keys) to compare. Use Actual size to look at the pixels 1:1. Look at faces, text and smooth areas such as sky.
4. Choose:
   • Compress – writes exactly the file you saw, then every WordPress size of the image. A table shows the result for each file.
   • Keep the original – takes the image off the list and changes nothing.
   • Try a gentler result – shows a version at a higher quality (a larger file), for an image where you can see a difference.

The Target size (default 50 KB, the size the Page Audit's image-weight finding aims for) and, for JPEG and PNG images, the copies to make can be changed above the preview; click Preview again after a change.

When the target cannot be reached without visible loss, Caching Out says so and shows the smallest size it reached: that image needs to be made smaller in pixels (in your image editor, or by the theme using a smaller WordPress size). Image compression never resizes an image.

A compressed image leaves the Selected for compression list by itself.

Which images it compresses

• JPEG – recompressed at the highest quality that lands under your target, plus WebP / AVIF copies.
• PNG – compressed losslessly only, so savings are usually small; the WebP / AVIF copies are where a PNG gains most.
• WebP – recompressed in place at the same URL. Many sites already store WebP images (converted by another plugin or exported that way); Caching Out works from its exact backup, never from a file it already compressed, so quality is not lost over repeated runs. Because a converted WebP has usually been compressed once already, look at the before / after view before you keep the result.

It does not compress:

• animated WebP (re-encoding would drop frames) or lossless WebP (it would stop being lossless);
• GIF and SVG files;
• images that belong to your theme or a plugin rather than the Media Library (the Page Audit says so on the finding);
• images stored outside wp-content/uploads (for example on an offload service).

Caching Out compresses the images you choose, one at a time. It has no bulk or automatic compression.

What it will not do

• Make a file larger. A file is replaced only when the result is smaller; a saving under 10% is skipped and the file is kept as it was ("Kept: would not be meaningfully smaller").
• Change pixel dimensions or URLs.
• Compress twice. A compressed image says so; to compress it to a different target, restore it first. Compressing always starts from the original, so quality is never lost over repeated runs.
• Touch a file that changed after it was compressed. If a size was regenerated (for example by a thumbnail regeneration plugin) or the file was replaced, Caching Out leaves that file alone and the screen says so. On Restore, the old backup of such a file is set aside in backup/.superseded/ rather than written over the new file.

Restoring

• One image: open it on the Image compression tab (or use Restore in the Media Library) and click Restore the original.
• Every image: Restore all on the Image compression tab. It shows how many images and files will be put back, asks you to confirm, then restores them one file at a time.

Restore checks each backup against the fingerprint taken when it was made, puts the original back, and deletes the copies.

Where the files are

Everything lives in one folder, wp-content/cachingout-images/ (per site on a multisite network):

• uploads/... – the WebP and AVIF copies, at the same path as the original with .webp or .avif added.
• backup/uploads/... – the exact original bytes of every file that was replaced.
• backup/.superseded/ – backups set aside because WordPress, or another plugin, changed that size after it was compressed. Caching Out never deletes these; remove them yourself once you are sure you do not need them.

On Apache and LiteSpeed the backup/ folder is blocked from the web automatically. On NGINX, add this to your server block so the backups cannot be downloaded:

location ^~ /wp-content/cachingout-images/backup/ { deny all; }

The folder also holds a small .htaccess that makes Apache and LiteSpeed send the copies as image/webp and image/avif (some older Apache builds would otherwise label them as JPEGs). On NGINX, if your server's mime.types lacks either type, see Server configuration.

Deleting an image from the Media Library deletes its copies, its backups and its record too.

The page cache and your CDN

The smaller file keeps its URL, so cached pages need no change to use it. After the first compress of an image picked on the Page Audit, the audited page is purged so the next visit picks up the new copies. When a run replaces or removes copies that already existed (a Restore, for example), the whole page cache is purged, because any cached page might point at them; Restore all purges once rather than after every image. If the Cloudflare or Varnish add-on is on, the changed image URLs are purged there too.

Visitors and CDNs that already downloaded the old file keep it until it expires. That is harmless: it is the previous, valid image.

On shared hosting

Each request handles one file, so a slow server compresses a large image in several short steps instead of one long one. Before each file, Caching Out checks that PHP has enough memory for an image of that size; if it does not, the image is refused with a plain message rather than taking the site down. Raise WP_MAX_MEMORY_LIMIT, or use a smaller image. Very large images still compress but show their sizes only, without the before / after pictures.

When you uninstall Pro

Uninstalling Caching Out Pro removes its compressed copies, its records and its list of selected images, but keeps the backups of any image you have not restored, in wp-content/cachingout-images/backup/. Use Restore all on the Image compression tab first if you want every original back; delete that folder yourself once you no longer need it.

If you install Pro again later, an image whose backup was kept shows as changed since compression and cannot be compressed again until you remove its backup folder; the backups themselves are left untouched.

Frequently asked questions

Why does "Compress this image" not appear on my Page Audit finding?

The finding says why in its text: the image belongs to your theme or a plugin rather than the Media Library, it is an animated or lossless WebP, a GIF or an SVG, or Pro is not switched on yet. Run the audit again after updating to Caching Out 1.25.0 and Caching Out Pro 1.25.0: results saved by an older version do not carry the button.

I use another image optimisation plugin. Can I use both?

Yes, on different images. Avoid compressing the same image with two tools: each keeps its own backup, so restoring through one would not know about the other. If another plugin changes a file after Caching Out compressed it, Caching Out leaves that file alone.

I compressed one image. Why did several files change?

When you upload an image, WordPress also saves smaller copies of that same picture (thumbnail, medium, large, and any sizes your theme or WooCommerce adds), and serves them to phones and in lists through srcset. Compress does all of them together, at the quality you approved for the full-size image, so a phone gets a smaller file too. The screen says how many sizes there are before you click Compress, and the results table lists each one. They are all the same picture; no other image is touched. Restore puts every one of them back.

Can it compress all my images at once?

No. Caching Out compresses the images you choose, one at a time, each after you have seen it before and after.

Is anything uploaded?

No. Everything happens on your server; your images are never sent anywhere.

Does Safari get the smaller image?

Yes. The optimised original keeps its URL and its format, so every browser downloads the smaller file. Safari does not announce WebP or AVIF support on page requests, so it receives the original format, which is the file Caching Out made smaller.

What does "Try a gentler result" do?

It shows the same image at a higher quality, a larger file, in case you can see a difference in the first result. Compress then writes exactly that version.

How do I undo it?

Restore the original on the Image compression tab or Restore in the Media Library puts the exact original bytes back. Restore all does every image. Restore works with or without a licence.