Free is always good, but its much better when its free artwork from the Black Library. There is two sets of these, which apparently several friends missed, so make sure you check out both links below.
From the Black Library.......
Free Artwork
http://www.blacklibrary.com/warhammer-40000/12-months-of-40k
Free Digital Codex Artwork
http://www.blacklibrary.com/games-workshop-digital-editions/