After frosting the cakes, I drew the image on with a toothpick. (I printed an image I wanted to use. Then traced the outside of the image on the cake. I freehand drew the inside features.) Then I colored in the image with black Wilton frosting, using a thin writing tip. I accented with red and blue frosting.
Toothpicks are a great tool to use when decorating cakes. Frosting can be maneuvered with the toothpick as well if it doesn't lay exactly the way you want it.