Photos are usually far bigger than they need to be. Drag the quality down until the size is where you want it — the page shows what you actually saved, not an estimate.
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.
Usually 60–90% for a photo straight off a phone or camera. The page shows the true before and after, and if a file is already as small as it can go it says that rather than pretending.
At 70% quality most people cannot tell on a screen. Below about 50% you will start to see it in flat areas like skies.
Yes — set the longest side and it is scaled down before compressing, which usually saves far more than quality alone.
Everything here is free and works the same way — in your browser.