This Washington Post editorial attempts to answer this very question, regarding lawsuits over the use of copyrighted images in Twilight fan material. Following an apt quote from Tolkien, the author suggests:
Films such as the Twilight saga resonate because they show us complex characters grappling with big issues. …
Pictures, videos and slogans on T-shirts are tools of modern expression, and with a phenomenon as omnipresent as Twilight, fans should be free to engage, manipulate, remix and remake. Free speech is just too important for anything less.
Fans of any kind of cultural artifact, Twilight or otherwise, should agree with that.
via Washington Post – The Twilight copyright saga: Forbidden love and forbidden T-shirts.