Questions
Sign on plain white paper with a black pen, put it on a flat surface in daylight, and shoot from straight above without your shadow across it. Crop close in the frame. The “Clean up the paper” step then turns the paper white and the ink dark, even if the photo came out grey.
A 140 × 60 px image holds very little — a clean signature at that size is usually 3–8 KB even at maximum JPEG quality. Sharpmonk pads the file to the minimum with a standard JPEG comment block, which every viewer and portal decoder skips.
It only changes tone: shadows and grey paper become white, ink becomes near-black, and the edges stay smooth. The strokes themselves are untouched, so it still matches the signature you'll sign at the exam. You can switch it off.
Not for IBPS: its notification says signatures in capital letters are not accepted. Sign the way you normally do.
Doing a full application? The exam pages — IBPS, SBI, UPSC, SSC, RRB NTPC, NEET UG and JEE Main — size every upload at once.