These are user-created backdrops/screens that are either in the public domain, or creative commons. They are all free to use in your game. If you are in doubt as to whether or not you need to credit the creator, it's best to get in touch with him/her.
Vikings of Midgard
Taoki's modified Vikings
FnrrfYgmSchnish's 8-Bit Backdrops
By FnrrfYgmSchnish. These go along with the matching maptiles posted on the Free Tilemaps page.
Also available for download from SlimeSalad.