Right now I have my background set to randomly show any of about 35 New England Patriots related images (for the NFL playoffs, haven't switched out yet after our Superbowl win).
Normally though, basically for the past 8 years I think, I have my background set to switch every 60 seconds randomly picking from a folder of 58 different pics with the general theme of "black/dark background with multicoloured accent/objects/graphics"
I suspect I'll always keep these around till they are no longer fit graphics wise to be used.
Like this:
http://i.imgur.com/s72afNM.jpg
http://i.imgur.com/q1IvKYO.jpg
http://i.imgur.com/HRyr8qo.jpg
And as an actual non-linked one: