Compress an image to 20 KB

Because at 20 KB there is no alternative. JPEG quality alone bottoms out long before a full-size photograph reaches 20,000 bytes, and grinding past that point produces a smeared picture rather than a small one. Reducing the pixel count lowers how much detail has to be stored in the first place, which buys far more than another turn of the quality dial. The result panel always names the dimensions it settled on.

Almost always a photograph or signature attached to an application form - exam registrations, job portals and government submissions are the common cases. Those forms tend to state a pixel size as well as a byte ceiling, so it is worth checking whether yours does before you upload: this page aims at the byte ceiling, and if your form also names exact dimensions the application-form tool handles both together.

Usually not, and it is better to know that before uploading. Text survives compression badly because its sharp edges are exactly what a lossy encoder discards first, and at 20 KB a full page of type will typically blur into something a human can guess at and a machine cannot read. If the form allows 50 KB or 100 KB, use it; if it does not, crop to just the part that matters rather than shrinking the whole page.

If there is anything you do not need in frame, yes - and it helps more than any setting on this page. Cropping removes pixels that would otherwise have to be stored, so the same 20 KB gets spent on a smaller area and the part you care about keeps more detail. A passport-style headshot cropped tight to the face will look markedly better at 20 KB than the same face inside a full-body photo.

The tool stops at the quality floor and reports the smallest size it reached, rather than continuing down into results nobody would accept. You then have a real choice: take that file, pick a larger target, or crop first. What it will not do is hand back something above your limit while implying the limit was met.