Image tools

Which format you actually want#

JPG stores photographs in a fraction of the space of anything lossless, because it discards detail the eye is bad at noticing. That trade is excellent for a photograph and poor for a screenshot: flat colour and sharp text are exactly what its compression handles worst, which is why a screenshot saved as JPG comes back with grey fuzz around the letters.

PNG is lossless and supports transparency, which makes it right for logos, icons, diagrams and screenshots, and wasteful for photographs. WebP does both jobs and lands roughly 25 to 35 per cent smaller than either at comparable quality, with support in every current browser. The usual reason not to use it is a tool further down your pipeline that still cannot read it.

Converting between lossy formats compounds the loss. A JPG turned into a PNG is a lossless copy of an image that has already been damaged: the file gets larger and the artefacts stay. If you have the original, convert from that rather than from an export.

What running in the browser changes#

These tools use the canvas and File APIs, which every browser has had for over a decade. The page reads your file into memory, redraws it at the size or in the format you asked for, and hands the result back as a download. No request ever carries your image, which you can confirm by opening the network panel and watching it stay empty.

That changes three things you can feel. A batch of twenty images is limited by your processor rather than your connection, because there is no upload to wait for. There is no file size cap of the kind an upload form imposes, though the tab still has to fit the decoded image in memory, which a desktop manages easily and an old phone may not. And once the page has loaded, it keeps working with the network off.