This is the laziest route (although you can always amend what you get later)

A number of suppliers (see links below) will take your favourite photo and print it out using edible ink which they'll then put on a cake for you. If you're a smooth talker, perhaps you can just get the printout to add to your own cake.

Unless you're a very fast talker, most of the providers will refuse to print anything to risque for you. However once you have the printout, the cake is your photoshop.

Moustaches, glasses, freckles, go wild with the paintbrush!