Time for a change then?
Whenever i've been off road the wallpaper is usually updated not long after (suitable photos permiting).
Where did the wallpaper come from and did you specifically saved onto you computer. It may be stored in some random file used by windows when down loading etc.
As a preventative measure for future wallpapers, I save the wallpapers in a suitable named filed in 'my pictures' so if theres a problem you know where to find it.
There may be ways to retrive it, but they would be beyond my experiance.
