How image compression works
Photographs are large because every pixel stores colour information. Lossy formats such as JPEG and WebP shrink files by discarding detail that the eye is least likely to notice, using a quality setting to decide how much. Lowering quality from 100% to about 75% typically reduces a photo's file size dramatically with little visible change, whereas going below 50% starts to show blocky artefacts around edges. PNG is lossless: it keeps every pixel exactly and therefore changes size only a little, but it supports transparency.
What this tool does
When you choose a file, your browser decodes it and draws it onto an invisible canvas. The canvas is then exported again in the format and quality you picked, and the result is offered as a download. Because the work is done by the image engine built into your browser, there is nothing to install and no upload, which keeps private pictures private and avoids slow transfers. If you give a maximum width, larger images are scaled down proportionally first, which usually saves more than lowering quality alone.
Choosing settings
- Photos for the web: JPEG at 70% to 80%, or WebP at the same level for even smaller files in modern browsers.
- Screenshots, logos and graphics with flat colour: PNG, since JPEG can blur sharp edges and text.
- Width: a phone screen rarely needs more than about 1,200 to 1,600 pixels across. Camera images of 4,000 pixels or more are far bigger than most pages use.
Things to watch for
Converting transparent PNGs to JPEG fills transparent areas with white, since JPEG has no transparency. Re-compressing an already compressed JPEG loses some quality each time, so keep the original. If the "compressed" result is larger than the original, which can happen with images that were already optimised, the table says so and you should keep the original file. Image metadata such as camera model and GPS location is not copied into the new file, which is a privacy benefit.
Performance and privacy
Large images use memory while they are processed, so on a phone it is best to handle a few files at a time. All processing is local, and the files are never sent to a server. The table links point to temporary in-memory files that disappear when you close the page.