How do I register or trademark a picture on a tee shirt?

Hi! Thanks for the question.

Please take note of the important disclaimer below that the answer
here is general information only and is not a substitute for informed
professional legal advice.

Our first link provides complete instructions on how to register for a trademark.

?What must the application include??
http://www.uspto.gov/web/offices/tac/doc/basic/appcontent.htm


However, I feel that more than the trademark, in this case you would
rather register for a copyright. You can view the instructions on how
you can register a visual art work for a copyright.

?Visual Art Works?
http://www.copyright.gov/register/visual.html

Search strategy used:
Searched at the following federal government websites
a. http://www.uspto.gov/ 
b. http://www.copyright.gov/

I hope these links would help you in your research. Before rating this
answer, please ask for a clarification if you have a question or if
you would need further information.
                                                          
                              
Regards,                              
Easterangel-ga                              
Google Answers Researcher