How browser compression works
The selected image is decoded into a canvas, optionally resized, and encoded again in your chosen format. Nothing is posted to a server. Closing this tab releases the working data.
Choose a useful format
WebP usually works well for web photos and screenshots. JPG is widely compatible for photographs but does not keep transparency. PNG preserves transparency and sharp graphics, although it may produce a larger file.
Quality is visual, not absolute
Start near 80% and inspect fine text, hair, and sharp edges. A smaller dimension often saves more bytes than aggressively lowering quality.