Discstalker states they don't do copyright materials. However, if you know how to do a bit of photoshop, you can change enough to make it your own under some of the creative exclusion clauses to copyright materials so it is no longer copyright so long as the derivative work is for you only. Meaning don't ask discstalker to print out a bunch of some image you modified because they won't.
A good example is my avatar. See my post history. I took a picture that was copyrighted, but added in a T pad, some extra lines, completed the "disc" in the image, added a top, border, writing, and a basket to the image. I changed enough that it is a new derivative work that while I couldn't sell it, is not protected the same way as the original work is.
http://en.wikipedia.org/wiki/Derivative_work
So by adding in those elements to the original work, I made substantial modifications and put in my personality to the original work. Technically as a derivative it is my work now. I could copyright it and resell it.... but it can get fuzzy a bit. So long as I am using derivative work for personal use it will be just fine.
So if you want to do a Warhammer 40K print as a derivative work, I suggest improving your photoshop skills. Take that chaos armor image. Instead of holding a gun, have him hold a disc. Now's it is substantially changed and reflects a different personality as a derivative work.