Fill in one side and the other follows so nothing gets stretched. Fill in both and the picture is fitted inside those bounds.
This page loads no libraries at all — the work is done by the browser itself. Your file is read on your own machine and never sent anywhere — open your browser's network tab and watch while you use it.
Not unless you ask it to. Leave "keep proportions" on and the shape is preserved.
You can, but a small picture blown up just looks soft — there is no detail to invent.
Everything here is free and works the same way — in your browser.