I've been a fan of 40k since the first Dawn of War and I shortly after started reading the books. It would be a dream to do some official art for these guys.
Games Workshop owns the Warhammer and all its affiliated works.
I was going to recommend that you contact Fantasy Flight Games. They have the licence for utilizing GW intellectual property in roleplay and boardgames and employ a lot of freelance illustrators for their publications. Although your illustration is a little adrift from official canon, your composition and attention to detail is up there with some of their professionals.
I took some creative license with the space marine helmet and I decided to go without the tyranids' crab-like armor to create a greater juxtaposition. I'm going to create another piece or two and try and contact FFG for some possible work.