Make images smaller without uploading them

Images are almost always the largest thing on a page, and image weight is the single most common reason a site feels slow on a phone. It is also the easiest to fix, which is why it is worth doing before anything more sophisticated: no amount of code splitting compensates for a two megabyte hero photograph.

The judgement is where to stop, and the useful thing to know is that the relationship is not linear. Going from maximum quality to about eighty percent typically removes most of the file size and almost none of the visible quality, because the first thing compression discards is detail the eye was not using. Below roughly sixty, artefacts start appearing in gradients and around edges, and they are most obvious in exactly the images people care about: skies, skin, and anything with a soft transition.

Format matters as much as quality. Photographs belong in JPEG or WebP; screenshots, diagrams and anything with flat colour and hard edges belong in PNG or WebP, because JPEG's artefacts cluster around sharp edges and make text look smudged. A screenshot saved as a low quality JPEG is the classic example of compression applied to the wrong format.

The before and after comparison is the point of doing this deliberately rather than accepting a default. Compression is a trade, and the only sensible way to make it is to see both sides at the size the image will actually appear. An image judged at full screen and published in a thumbnail was over-thought; the reverse is worse.

Worth remembering that compression is lossy and one way. Keep the original somewhere, because a compressed image recompressed a second time degrades again, and the version that ends up in the content library tends to become the master by accident.

It runs in your browser, which for a batch of client photographs or product images matters commercially even where it does not matter legally. There is no upload, no queue and no cap on how many you do.