the wallpaper appears to be stored in the registry here: HKEY_USERS\S-1-5-21-682003330-1417001333-839522115-1004\Control Panel\Desktop
on my machine.
*EDIT*
although it looks like SetSystemParametersInfo() combined with SPI_SETDESKWALLPAPER might be a safer bet
although if you're going for a registry approach, HKEY_CURRENT_USER\Control Panel\Desktop\wallpaper looks better than my first registry entry
on my machine.
*EDIT*
although it looks like SetSystemParametersInfo() combined with SPI_SETDESKWALLPAPER might be a safer bet
although if you're going for a registry approach, HKEY_CURRENT_USER\Control Panel\Desktop\wallpaper looks better than my first registry entry