A useful hack!
For 10,000,000 years, I've had a scan of my sig on my computer. When I get sent a form for a signature, I open it in the GIMP, open the signature JPG, paste the latter into the former, set the layer to "multiply" and align it.
This morning, I pasted that same JPG as a *brush*. Now I just select that brush and click once on the signature line!
This is incredibly obvious, but it never occurred to me, and maybe it's useful for you, too.