I think two other things which increase realism are some slight blurring, presumably due to colour bleed, and old tats especially seem to go sort of blurred round the edges of any block of colour;  and secondly the imperfections inherent in the process. I don't suppose live skin is particularly easy to colour, there are bound to be small areas of less pigment, some areas which the tattooist never coloured at all, and so on. A problem with using clip art, especially if created originally in a vector package, is that it looks too perfect for a real tattoo.

Steve