Get a scanned signature under the 20KB limit that application forms ask for. Free and private.
Upload fields for signatures normally enforce three things at once: the file size, the pixel dimensions, and the format. A file can be under the size limit and still be rejected for being 2000 pixels wide, or for being a PNG when the form said JPG.
The usual specification is 140 × 60 pixels, 10–20KB, JPG. Some bodies state it as 4.0cm × 2.0cm instead. Either way the practical target is the same: small, sharp, and saved as a real JPEG.
Requirements change between notifications, and different posts under the same body sometimes differ. Always open the official notice or the upload page itself and check the numbers before you submit — the figures here are the ones that have applied most recently, not a substitute for the notice.
Sign in black ink on plain white paper. Blue ink is accepted almost everywhere but scans with less contrast, and lined or off-white paper makes the background harder to clean up.
Photograph or scan it, then crop tightly — just the signature with a small margin. This single step usually does more for the file size than any amount of compression, because you stop storing the rest of the page.
Then drop it in above. The signature is compressed down under 20KB automatically and saved as a JPEG, which also fixes the common problem of a phone having saved it in a format the form will not accept.
The most common rejection reason is not size at all — it is legibility. Bodies like the SSC state outright that blurred or miniature signatures are rejected, so a signature that has been squashed to fit by brute force will fail even if the file size is correct.
The order that avoids this is: crop, then fix contrast, then compress. Compressing a full page photo straight down to 20KB produces exactly the blurred, tiny result that gets applications sent back.
Most Indian exam and banking forms ask for roughly 140 × 60 pixels at 10–20KB, in JPG format. A few ask in centimetres instead — around 4.0cm wide by 2.0cm tall — which comes to about the same thing.
Either works. A phone photo in daylight next to a window is usually cleaner than a cheap scanner, as long as your hand is not casting a shadow across the page.
Almost always a contrast problem rather than a compression problem. If the paper is grey rather than white in the original, compression makes the ink and the background converge. Brighten the image first, then compress.
Most banking and exam bodies specifically require a running cursive signature and reject block capitals. Use the same signature you would use on a cheque.