Surprised this hasn't been answered for you by now...
Just right click anywhere on the desktop, and at the bottom of the menu is Desktop Settings.
Click on that, and about halfway down you'll see the word Image.
Under that is File:, and at the right is a folder icon.
Click on it and you'll get a list of wallpapers (backgrounds) to select from.
There is even a preview pane on the right.
Good luck, it's pretty smooth.
Linux Mint Debian Edition (LMDE)
HP Omni 220-1050XT, Intel(R) Core i5 2400S Quad CPU @ 3.3 GHz, 6Gb DDR3 Ram, 1Tb HD,
AMD Radeon HD 6450 Graphics 1GB, 21.5" LCD, Beats Audio