How it works
Each image is drawn to a canvas, optionally resized so it is no wider than the max width, and encoded again with your browser's built-in encoder. If a converted file comes out larger than the original, the table says so, and you can keep the original instead.
FAQ
Which format should I pick?
WebP is the safe choice for websites and store pages: much smaller than PNG or JPEG and supported everywhere. AVIF is smaller still but slower to encode, and only appears if your browser can create it. Keep PNG for pixel art and anything that must stay lossless.
What quality setting is good?
Start around 80. Photos and painted art usually look the same at 70 to 85. Lower it until you can see artifacts, then step back up.
Does PNG use the quality slider?
No. PNG is always lossless, so the quality slider is ignored. Resizing still makes PNG files smaller.
Is image metadata kept?
No. Re-encoding drops EXIF data such as camera details and GPS location, which also saves a little space.