Compression keeps a photograph recognizable while storing less data. A JPEG quality around 70 to 80 is often enough for a web page. Fine hair, small text, and a hard product edge are the first places a low setting shows, so look at the result at the size it will appear.
PNG stores pixels without throwing detail away. The quality slider does not shrink a PNG. A screenshot, logo, or flat graphic should usually stay PNG. A photograph that is currently PNG will usually get smaller as JPEG or WebP.
WebP is a good photograph format when the place you will upload the file accepts it. This site encodes the file in your browser and shows the real output size. If the new file is larger, it says so. The percentage is the measured difference, not an estimate.
1. Open Compress and choose one JPEG, PNG, or WebP under 10 MB.
2. Pick JPEG or WebP for a photograph. Leave a logo on PNG.
3. Move quality, run the tool, and compare the two file sizes.
4. Download only if the result is small enough and still looks right.