Image compressor
Nothing is uploaded: every image is processed in this browser
Image compression re-encodes a picture at a lower quality setting, discarding detail the eye is least likely to notice. Dropping a JPEG from quality 95 to 78 typically halves the file with no visible difference. Capping the longest side saves far more again.
How to compress images
Resolution usually matters more than quality. A 4000-pixel photograph displayed at 800 pixels wide is carrying twenty-five times more data than it can show, and capping the longest side removes that waste before the quality setting does anything at all. Compress first by dimension, then by quality. Doing it the other way round means throwing away detail from pixels you were about to discard anyway. For photographs, WebP typically produces a file about 25–30% smaller than JPEG at matching quality.
Questions
As many as your machine will hold. There is no server, so there is no upload limit and no queue. The only constraint is memory.
No. Every image is decoded, resized and re-encoded by your own browser. Nothing leaves the device.
Around 78 for photographs. Below about 60 the artefacts become visible on flat areas and edges.
PNG is lossless, so the quality slider does nothing for it. Convert flat graphics to PNG and photographs to JPEG or WebP.
Browsers block repeated automatic downloads, so a loop would silently drop most of a batch. One ZIP always arrives complete.