How to compress images in WordPress

When you upload an image, WordPress generates a copy at every registered size. A default install creates a few; an average theme with a page builder can register a dozen or more, and plugins add their own.

So a single 8 MB upload does not cost 8 MB — it costs 8 MB plus every derivative, all generated from that oversized source, all sitting in your uploads folder forever. Multiply by a few hundred posts and the media library becomes the largest thing in your backups.

Uploading a sensibly sized file makes every one of those copies cheaper at once.

Modern WordPress scales very large uploads down automatically, keeping the original as a separate file and using the scaled version as the "full" size. The threshold sits at roughly 2560 pixels.

This is a sensible safety net, but it is not optimisation: the scaled file is still re-encoded from a huge source at whatever quality WordPress chooses, and the original is retained on disk. Doing the resize yourself gives a better result and does not leave a copy behind.

WordPress outputs a srcset listing the available sizes, and the browser picks based on the layout and the screen. This is genuinely good behaviour — it means a phone downloads a small file — but it only works if the appropriate sizes exist.

Two common failures: a theme that outputs the full-size image directly in a template, bypassing srcset entirely; and a page builder that sets a background image in CSS, where responsive images do not apply at all. Both show up immediately in the network panel.

An optimisation plugin is worth having for two things: converting an existing library to WebP in bulk, and serving the right format to each browser. Some also offload delivery to a CDN, which is a separate and often larger win.

What a plugin cannot do is make a badly chosen original good. Compression is lossy — a plugin re-compressing your already-compressed upload is adding a second lossy pass, not recovering anything. Get step one right and the plugin has an easy job.

Be wary of plugins that compress by uploading your images to the vendor's servers, particularly if the site handles anything sensitive. That is a data-processing relationship, not just a plugin.

• Disable image sizes your theme does not use, so new uploads stop generating them.
• Regenerate thumbnails after changing sizes, or old posts keep pointing at the old files.
• Set explicit width and height on images so the browser reserves space and the page does not jump as it loads.
• Leave lazy loading on for images below the fold, and off for your hero image — lazy-loading the largest visible element delays the very thing your page speed is measured on.

Menu names and options move between releases. If a step below does not match what you see, the underlying capability is usually still there under a nearby name — and the browser tool on this page works identically on every version.