Copyrights are very important in the world of merchandising

Copyrights are very important in the world of merchandising. The laws regarding copyrights are confusing even to experts, so it is no wonder beginners often make mistakes. Some common misperceptions include:

But it's public domain. Are you sure? Just because it is found on the Net, lacks a (c)copyright marking, or was published in the newspaper does not mean it's public domain.
I can use it, because I got it from a graphics program I paid for. Most graphics packages include clip art and photo collections - but the license agreements are usually for non-commercial use only. Even if they do grant rights for commercial use, use on merchandise is frequently treated differently and still requires another, explicit grant (and usually for additional fees).
I didn't use the whole thing, so it's "fair use." "Fair Use" is a copyright term that applies to derivative works, usually for educational purposes. It rarely applies to commercial products for sale, and courts usually favor the author.
It's parody. Parody laws are contentious. Even if you win a court case, it can cost a lot of money. That's why professionals like Weird Al Yankovic get permission before they publish a song. Like other areas of copyright law, non-profit parody is treated very differently from for-profit parody. It also covers copyright, but does not cover trademarks, so a "parody" of a corporate trademark could very well wind you up in court. Please read the parody link provided.
What about freedom of speech? Freedom of speech is a constitutional protection guaranteeing that the government will not oppress your political speech. It does not cover freedom of merchandising, nor freedom of having your speech for sale in a T-shirt shop.

This is complicated law, and big businesses frequently defend their intellectual property with expensive lawsuits. If you have any doubt, get legal advice before selling something, or think about using a different design that does not violate someone else's copyrights.