Use registry to change to whatever wallpaper you like temporarily. You can edit registry using total commander or regedit.
Code:
[HKEY_CURRENT_USER\Software\HTC\Manila]
"Co0kie.Wall.AllPortrait"="\Windows\Stwater_480_800.jpg"
"Co0kie.Wall.AllLandscape"="\Windows\Stwater_800_480.jpg"
Instead of "\Windows\Stwater_480_800.jpg", type in your wallpaper path. Eg: "\My Documents\My Wallpaper\Road.jpg"