It seems like a minor technical detail, but the file format genuinely changes how a signature looks once placed on a document — and the difference is usually obvious the moment you compare the two.
PNG supports transparency; JPG doesn't
A signature saved as PNG can have a fully transparent background, so only the ink strokes themselves appear on the document. JPG doesn't support transparency, which means a signature exported as JPG carries a solid white (or colored) box around it.
That white box is the whole problem
Placed on a document with any background other than pure white — a letterhead, a colored template, another element behind it — a JPG signature's box becomes visibly obvious and looks unpolished, exactly the opposite of what a signature is meant to convey.
PNG also compresses without quality loss on flat-color images
A signature is mostly simple lines on a transparent field, which PNG handles efficiently and losslessly — the strokes stay crisp rather than picking up the slight blur or artifacts JPG's compression can introduce on fine detail.
When JPG might still be fine
If a document genuinely has a solid white background everywhere the signature will sit, a JPG can look acceptable — but it's a narrower use case than most people expect, and PNG works correctly in every case, so there's rarely a reason to choose JPG deliberately.
Always double-check before sending
Placed the signature and it looks fine over your specific document background? Good. If there's any visible box or mismatched color around it, that's the file format telling you it should have been PNG.