I’ve been getting great backgrounds from Digital Blasphemy for years! The creator of these incredibly detailed background images is Ryan Bliss and he does this for a living. As a temptation to join, he graciously presents a rotating list of FREE images for download here.
Currently, he is posting some of his Halloween renders on the site like the one shown above.
If you like his work, I highly recommend joining and getting access to more versions of the pictures for double and triple monitor setups as well as smartphone versions and even animated ones!
Digital Blasphemy has great (and geeky) backgrounds. They are worth checking out.