KB, pixels and DPI: what the form is asking for
Upload instructions mix three different measurements, and most rejected uploads come from confusing them.
Pixels: how big the picture is
A digital picture is a grid of coloured squares called pixels. "200 × 230 pixels" means 200 squares across and 230 down. More pixels means more detail. A phone camera produces pictures of around 4000 × 3000 pixels, far more than a form wants.
KB: how heavy the file is
KB (kilobytes) measures storage. It depends on the number of pixels and on how hard the picture is compressed. Two pictures with the same pixel size can have very different KB: a plain signature on white paper compresses to almost nothing, while a detailed photo does not.
This is why forms give a range such as 20 to 50 KB. Too large and it wastes their storage; too small and the picture is probably too blurry to identify you.
DPI: how it prints
DPI (dots per inch) links pixels to paper. A 276-pixel-wide photo printed at 200 DPI is 1.38 inches, or 3.5 cm, wide. DPI does not change how the picture looks on a screen, but some forms read it to check the physical size.
Putting it together
When a form says "3.5 × 4.5 cm, 200 DPI, 20–50 KB, JPG", it is asking for a 276 × 354 pixel JPG whose file weighs between 20 and 50 KB. Set the pixels first, then adjust the compression until the KB fits. The resizer on this site does both steps for you.