How resizing works
Your image is decoded, redrawn at the new size on a canvas, and saved again in its original format. All of this happens in your browser: the file is never uploaded to a server. JPG, PNG and WebP images up to 25 MB are supported. The file is checked by its actual content (its format signature), not just its name or extension, so a renamed non-image file is rejected with a clear message.
Keeping the aspect ratio
With "Lock aspect ratio" on, you only need to enter a width; the height is calculated so the picture is not stretched. For example, a 4000 × 3000 photo resized to a width of 1200 pixels becomes 1200 × 900 pixels. Turn the lock off if you deliberately want different proportions, for example to fill a fixed-size slot; the image will then be stretched or squashed to fit.
Scaling by percentage works the same way on both sides at once: 50% of a 1920 × 1080 image is 960 × 540 pixels.
Quality and file size
Making an image smaller discards pixels, so it usually reduces the file size. Making an image larger than its original size cannot add real detail; the extra pixels are interpolated from the existing ones, so enlarged images look softer. For the sharpest result, resize from the largest original you have.
Privacy
Because processing happens entirely on your device, the tool also works for images you would not want to upload anywhere.