OK, this is probably old hat to true Windows geeks, but I just figured this out: how to change a user's desktop bitmap at range. You need admin rights on the victim's user's PC. Launch REGEDT32 and open the registry on their system. In the HKEY_USERS window, open up the user (they'll be represented by a big long string of gobbledygook*), open up Control Panel, then inside that Desktop. Look for the key Wallpaper. Change the value of that key to the file name of your choice.

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* The user may need to be actively signed on for this to work: in REGEDT32 (the remote registry editor) it appears that the HKEY_USERS window shows what you would see in REGEDIT under HKEY_CURRENT_USER. I checked a system here that no one was signed onto, and the key with the big long string of gobbledygook had no entries in it. Hence, I think that the user probably does have to be signed on.