Image compression reduces the size of an image file so it uploads faster, loads quicker and takes up less storage.
There are two kinds. Lossless compression (PNG, some WebP) rebuilds the image exactly, with no quality loss. Lossy compression (JPG, most WebP) discards fine detail your eye is unlikely to miss, which makes files dramatically smaller.
Compressing images is one of the biggest wins for a fast website and for staying under email or upload size limits.
Try Compress Image to shrink a JPG, PNG or WebP without a visible drop in quality.
Lossy and lossless
Lossless compression finds redundancy and encodes it more efficiently, so the decompressed image is bit-for-bit identical to the original — PNG and lossless WebP work this way. Lossy compression goes further and permanently discards information judged to be perceptually unimportant, which is how JPG and lossy WebP achieve much smaller files. The distinction matters practically: lossless is safe to repeat, while every lossy pass degrades the image a little more.
Resize before you compress
This is the step most people skip and the one that saves the most. File size scales with the number of pixels, so halving an image's width and height removes about three quarters of them before compression has done anything. An image displayed 800 pixels wide gains nothing from being 4000 pixels wide — the browser downloads all of it and then discards the excess. Resize to roughly twice the display size, to stay sharp on high-density screens, and then compress.
Choosing a quality level
JPG quality between 75 and 85 is where almost nobody can identify the compressed version and where the savings are steepest; dropping from 100 to 85 often halves the file with no visible change. Below about 60, gradients start to band and sharp edges pick up halos. Photographs tolerate aggressive settings well. Screenshots, diagrams and anything containing small text do not, because lossy artefacts cluster exactly where edges are sharpest.
Why it matters beyond convenience
Images are usually the heaviest thing on a web page, and Google measures Largest Contentful Paint — how long the biggest visible element takes to appear — as a ranking signal. On most pages that element is an image. Visitors on weak mobile connections routinely abandon pages before an oversized hero image finishes loading. Compression is the cheapest available performance work: no code changes, no infrastructure, just smaller files.