Resizing without distortion
Every image has an aspect ratio: its width divided by its height. A 4000 x 3000 photo has a 4:3 ratio. If you resize it to 1200 wide with the lock on, the height becomes 900 automatically. With the lock off you can force any size, but circles become ovals and faces stretch, so only unlock it when the destination truly requires a different shape.
Downscaling throws away pixels and is generally safe. Upscaling invents new pixels by interpolation, so enlarged images look softer. This tool allows enlargement up to 400% for practical cases like icons, but it cannot add detail that was never captured.
Choosing an output format
PNG is best for screenshots, diagrams and images that need transparency. JPEG is best for photographs where small size matters more than perfect edges. WebP often beats both on size and supports transparency, and every modern browser can display it. The resizer works entirely on a canvas in your browser, so images stay private and the operation is usually instant even on a phone.