Interesting. Maybe Apple made it a file that restores itself, or whatever the term would be. The sleepdisk is like that...if you delete the sleepdisk image file under /var, it recreates itself, unless you make it an immutable file, which you could try.
Put this into terminal to lock the file once you set it to the png you'd like.
sudo chflags uchg /system/library/frameworks/appkit.framework/versions/c/resources/NSTexturedfull screenbackgroundcolor.png
I got this from :
http://forums.macrumors.com/showthread.php?t=1480259
To delete it, there is another terminal command in that link, enter admin password to delete locked file, then restore it to the default background image file that you saved somewhere in a safe spot.
Might work!