How image conversion works here
Pick your file, choose the output format and download the result. Your browser decodes the original image, redraws it on a canvas and re-encodes it in the format you asked for. Nothing is uploaded, so a 40 MB photo finishes as quickly as a thumbnail.
Formats behave differently and we never hide that. PNG is lossless, so it keeps every pixel and supports a transparent background, which makes it right for logos, screenshots and graphics with flat colour. JPG throws away detail your eye is unlikely to notice, which is why photographs end up several times smaller. WebP and AVIF are newer, compress harder than JPG and are supported by every current browser, but older software still refuses to open them.
When a conversion is lossy we run the encoder at a high quality you would struggle to tell from the original, and show the resulting file size next to each file, so you can see the trade-off rather than guess at it.