You're only drawing a background. As long as the scene isn't also distinctly part of a copyrighted or trademark universe created by the same owner of the rights to the character, you're not liable for anything the other person adds to it. If the person is trying to do a parody or satire piece with both the character and your background, then neither of you has anything to worry about.
Show me one example anywhere in American case law where someone has been prosecuted for "aiding and abetting copyright infringement".
I don't mean actually facilitating it, like they're saying of Kim Dotcom who is charged with actually providing the means for illicit file sharing, but by doing something as incidental as this. You almost might as well prosecute the art supply shop for selling Copics to the infringer.
Hence the sentence: None of this will likely ever get litigated, but it's worth noting.
Also, one should never underestimate how many charges can be leveled against just about everyone involved - especially when it comes to the sue-happy nature of folks such as the RIAA/MPAA and other rights holders.
They tend to have a shotgun approach to litigation. And in the case of Kim, sometimes "shotgun approach" is taken literally.