For a definitive answer to this question, you should consult a lawyer that deals in intellectual property issues. He or she can advise you on the specific legal issues and the options you have to help protect yourself.
So, with the caveat that I am not a lawyer and this is not legal advice:
While it may appear that this is okay, given the wide variety of user-created mods you can find for various games that incorporate the likenesses of famous people or characters from popular games, et cetera, this is generally still considered a violation of one or more intellectual property laws (depending on what one is actually doing, the legal jurisdiction involved, and so on).
By supplying only the tools to potentially commit infringement, and allowing users to generate the infringing content yourself, you might not be directly culpable for the infringement. That said, in many jurisdictions (such as the US) you can be sued for just about anything. If somebody found this post and traced it back to your game, for which somebody had made an infringing mod for, they could in theory file suit against you or include you in a lawsuit for being complacent or for intentionally facilitating the IP infringement.
If you look, in fact, you'll find that many of the "big name" games that support modding include clauses in their license agreements requiring you, the user, not to make content that infringes upon somebody else's intellectual property rights. For example, the Skyrim Creation Kit EULA includes this passage in section 1 ("Restrictions on Use"):
You shall not create any New Materials that infringe upon the rights
of others, or that are libelous, defamatory, harassing, or
threatening, and You shall comply with all applicable laws in
connection with the New Materials.
Thus, you should consult a lawyer to discuss your options and whatever necessary verbiage you should add to your own EULA to help protect yourself.