I'm thinking something like this:
http://www.petportraits.org/page1_we...grey_large.jpg
Except without the reigns or whatever that is on it and realisticish but not super detailed (not something that looks like it was drawn by hand or cartoony but at the same just solid black and white).
My shirt will look like this:
A PICTURE IS WORTH
[HORSE'S HEAD]
A THOUSAND WORDS!