Well Photoshop cs4, cs5 and it might work in cs3 - cs2.
Its called clipping masks, you take a brush in photoshop ( I usually use splatter brushes for the effect ), brush it in there ( once or twice, dont want the same looking stuff all over your sig ) then clipping mask an applied image of the sig ( works with practicly any other img ) and there you have it in a nutshell.
If you have photoshop, ill give ya some tuts on it.