Make it bigger without the mush
A network that invents the detail a plain resize can only smear. For small pictures only, and nothing is sent anywhere unless you ask for it.
Drop a small image here
Up to 512 pixels on the longest edge. Pasting works too. The first image downloads the 13.39 MB runtime and the 4.65 MB model.
Why the limit. The other tools look at a picture; this one rewrites every pixel into sixteen, so the work grows with the picture. A 512 pixel square takes around twenty seconds on a reasonable machine. A full HD frame would take minutes and more memory than a browser tab will hand out, so it is refused rather than promised.
Bigger picture that just needs more pixels? A plain resample is free and does that better. Use crop and resize. This one is for logos, avatars, pixel art and old thumbnails, where detail has to be invented rather than stretched.
Result
Changes apply right away
The network only knows four times. Two and three are that answer resampled down, which keeps the invented detail. Changing this does not run it again.
PNG by default: it would be a shame to invent the detail and then throw it away in the encoder.
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Preview
The preview draws without smoothing, so what you see is the result rather than the browser softening it again on the way to the screen.
Questions people ask
Is my picture uploaded to upscale it?
No. The AI model is downloaded to your browser and runs there.
How big can the picture be?
Up to 512 pixels on its longest edge, which it makes four times larger. Bigger pictures would take longer than anyone would wait on an ordinary computer, so the page says so instead of trying.