If you are looking for some inspiration have a look at this selection of skulls courtesy of morgue File. Please do not copy these from my site but rather follow the link to morgue File and download them from there after reading the morguefile license.
Follow this link for my skull t-shirt design photoshop tutorial.
I have commented out the links to morguefile for each of these images because morguefile seems to have changed its site structure so the links don't work any more. View Source to see what they were.