We aren't laywers at Print Aura but here is some information to get you started if you have trademark / copyright questions or issues relating to t-shirt designs or slogans.
United States Patent and Trademark Office
This site is a must visit for anyone who has any doubt as to the legality of printing a word, phrase or design on a t-shirt. You can do a search to see if something is trademarked but remember just because there are no results in the trademark database doesn't necessarily mean that its legal. Copyright, trademark, and intellectual property are extremely tricky. When in doubt always consult a lawyer to prevent future lawsuits from image violations.
US Copyright Office
Lots of resources on copyright in the US.
World Intellectual Property Organization
Read up on trademarks and intellectual property on the WIPO's website.
7 legal tips for your t-shirt business - by T-Shirt Magazine