Mentioning the OS might not hurt !
In Win98se: you can select the desktop background from any *.bmp under c:\Windows .
There is no "other copy" as far as I know: the Registry just points to the image. So, if it's corrupted, that's that unless you have another image around. Of course, if you used an M$ provided image, you can restore that... but you didn't indicate where your background originated.
To add a new candidate image for a background, just create it as a BMP, store it under \Windows, and use "browse" under Properties -> background .
Of course, you may not be talking about Win98se, but GIGO !!