My personal opinion is that these guys should relax, if you are not "ruining" his art/concept or do anything offensive with it and you give him credit and doesn't cash in on his work, then he/they should appreciate that some people like what he made and are helping to promote it. Imagine how many people have made south park, the simpsons or Disney stuff.

Besides that in regards to legal matters, im pretty sure (depending on where you live) that the lawyers have to proof that you have sold any of the drawings, if they want to nail you for that. Besides that you could always try to contact the copyright holder and ask for permission as other have mentioned, its might not necessarily be his doing, but most likely his publisher that's behind it without him knowing about it. And if he is a decent guy, and you explain to him that you are just a fan, he might give you permission to post them.