Why 20 KB needs a different approach
At 20 KB there is very little room. Lowering quality alone will rarely be enough for a large picture, because below a certain point the JPEG blocks become obvious before the size gets small enough. The tool therefore lowers the pixel dimensions as well, which spreads the limited bytes over fewer pixels and keeps the picture readable.
For small images, this is fine. A 200 by 250 pixel photo at 20 KB looks acceptable on screen. A 2000 pixel photo forced into 20 KB without resizing would look like a mosaic.
Signatures: the easiest case
A scanned signature is mostly white paper with a thin dark line. That kind of image compresses very well. Crop to the signature, increase contrast if the ink looks pale, and the result typically falls well under 20 KB with decent edges.
Sign with a dark pen on plain white paper with no lines. Ruled paper adds detail that costs bytes and looks messy after compression. Good lighting when you photograph the sheet also prevents gray shadows across the background.
What to expect from photos at this size
Face photos can work at 20 KB if they are small, around 150 to 250 pixels wide. Fine detail like hair strands and fabric texture will soften, but faces stay recognizable. If an upload field also specifies pixel dimensions, resize to those numbers first so the compressor does not have to guess.
Do not use a 20 KB version for anything beyond the upload field. Keep your original and make the small copy only for the form.
Using the output correctly
Most strict portals expect JPG or JPEG. If your source is PNG, the tool converts it to a lossy format to reach the target, and transparent areas turn white in JPG. Check the file extension before uploading, and make sure the name has no unusual characters.
Common 20 KB mistakes to avoid
The most frequent mistake is scanning at very high resolution. A 600 dpi scan of a small signature produces millions of pixels for a line that needs a few thousand. Scan or photograph at normal settings and crop tightly.
Another is photographing a signature with a flash. The glare creates bright patches that turn into noisy gray areas after compression. Use natural light from the side and keep the paper flat.
Finally, do not stretch a tiny image back up. If a 20 KB file is blown up to fill a larger space, it will look soft. Keep the small copy for the upload field only.