An image compressor makes a photo or graphic take up fewer bytes so it loads faster, attaches to email and fits upload limits. This one re-encodes each image in your browser at the quality you choose. JPG, WebP and AVIF are lossy formats: they throw away detail your eye barely notices, which is where the big savings come from. At the default 75% quality, a full-quality phone photo usually shrinks a lot and is hard to tell apart from the original. A before/after slider with a 3× zoom lets you check for yourself when you compress a single image.
You can keep each file's format or convert as you compress. WebP is usually smaller than JPG at the same visual quality and keeps transparency; AVIF is smaller still but takes a few seconds per photo to encode. iPhone HEIC photos are decoded in the browser and saved as JPG (or the format you pick). An optional longest-side limit, for example 2000 px, scales oversized camera photos down, which often saves more than any quality setting.
Honest limits: PNG is lossless, and this tool has no lossless PNG optimiser, so re-saving a PNG as PNG rarely gets smaller. For screenshots and graphics choose WebP (keeps transparency) or JPG (transparent areas turn white) for real savings. If a re-encoded file in the same format comes out bigger than the original, the original is handed back untouched. Compressed copies don't keep EXIF data such as camera details and GPS location.
