An image upscaler makes a small photo bigger and sharper at the same time. Simply stretching a picture makes it soft and blocky; here a model trained on photos fills in the extra pixels, so edges stay crisp and noise goes down. It helps with old photos, pictures saved from a chat, product shots that look soft on a listing, and images you need larger for a print or a slide.
You can increase image resolution 2x or 4x. 4x is for small photos up to about 1 megapixel, like an old phone picture or a thumbnail; 2x works for photos up to 4 megapixels. The result is capped at about 16 megapixels, and a larger photo is scaled down before it is enhanced. On a weaker phone or laptop the limits are lower; the panel shows which scales are available and how long it should take before you start.
Everything happens in your browser. The model, about 32 MB, downloads the first time and is kept for next time; the photo itself is processed on your device and never uploaded. That's why it isn't instant: on a computer without a graphics card, a 1-megapixel photo took about 41 seconds and a 2.7-megapixel photo at 2x about 89 seconds. A graphics card makes it faster, and a phone is slower than a laptop.
Be realistic about what it can do. It sharpens what is there; it doesn't invent what isn't. A blurry face a few pixels wide stays a blurry face, a heavily compressed image can keep some of its blockiness, and text that can't be read in the original won't become readable. For a slightly soft photo, it is a clear improvement.
The result downloads as a high-quality JPEG, or as a PNG if the image has transparency, with no watermark. It takes JPG, PNG, WebP and HEIC from an iPhone.
After upscaling, you can remove the background for a clean product shot, write a listing from the photo, or compress the file if it is now too big for an email.