Compress image

No. Compression happens in a Web Worker on your device using the jSquash WebAssembly codecs. Nothing is uploaded, no account exists, and you can watch the request counter on the homepage stay at zero while you use it. The site also works offline once it has loaded a first time.

That depends on the picture and on the Quality option, which defaults to 70. The tool reports the exact result next to the file, in the shape "photo.jpg: 10891 to 4597 bytes (58% smaller)", so you can compare rather than guess. Photographic JPGs usually shrink a lot at 70. A small, already-optimised file might barely move.

Because re-encoding can cost more than it saves. That happens with a high quality setting, or with a PNG that was already packed well. The tool does not hide it: the summary line says "12% larger" instead of a percentage saved, and you can simply keep your original. Lowering the quality number is the usual fix wherever the format is lossy.

It re-encodes them, but the PNG encoder is lossless and ignores the Quality option completely, so the saving is whatever the codec can find in the packing alone. If you need a real size drop from a PNG photo, convert it to WebP with Convert image instead. Quality reaches only the lossy formats here: JPG, WebP, AVIF and JXL.

Your name with a -min suffix and the extension re-derived from the format that was encoded, so PHOTO.JPG comes back as PHOTO-min.jpg. The format is kept, with one exception: a HEIC comes back as a JPG, since no wasm encoder writes HEIC. Drop a whole folder if you like; each file is handled separately.

Every format Convert image reads, decided by the file extension rather than by sniffing the bytes. A file it cannot place stops with a message naming your file and the formats that are readable. A HEIC comes back as a JPG, since nothing here writes HEIC. It also does not resize anything: the output has exactly the same pixel dimensions as the input. For fewer pixels, use Resize image.