Image resizer
Your images never leave this device
Resizing an image means redrawing it at a new pixel size. Reducing the dimensions is what actually shrinks a file: a photo at 1200 pixels wide is roughly a sixth of the data of the same photo at 3000. The quality slider then controls how hard JPEG or WebP compresses what is left.
How to resize images
Fit inside never crops and never enlarges beyond the box you set, which is what you want for a gallery. Fill crops to cover the box exactly, which is what you want for a square avatar. Stretch ignores the proportions entirely and is almost never what you want, but it is there because occasionally it is.
Questions
No. The browser decodes and redraws them locally. You can disconnect from the network and the tool still works.
JPEG for photographs, PNG when you need transparency or crisp flat colour, WebP when you want both and know your audience’s browsers support it.
PNG is lossless, so re-encoding a photograph as PNG can easily exceed the original JPEG. Choose JPEG or WebP for photographs.
Yes, as a side effect: the image is redrawn from pixels, so camera model, timestamps and GPS coordinates are not carried over.
Around 80 is the usual sweet spot for photographs. Below about 60 the artefacts become visible in flat areas such as sky.