quite extremely particular about my desktop/phone wallpaper, and I've got a large collection of wallpaper images organized by type, source, light/dark content and yes, cross referenced (using symlinks). I typically change the wallpaper every few days, even though the only time I ever look at it is immediately after I've logged in. I don't know why, but having a good desktop background is really important; having a less-than satisfactory one really irritates me.
Here are some sources of really great, high quality (and usually free) wallpapers on the internet :
1. Smashing Magazine (updated on the first of every month)
2. Lifehacker's Wallpaper Wednesday (updated every week)
3. Inkdryer (updated daily)
Lifehacker's Featured Desktop/Homescreen (which updates every Tuesday or so) also feature some really nice ones, so take a look at those if your desktop is in need of a visual overhaul.
While we're on the subject, here's my current home screen setup: