The wallpaper can be found in the Windows registry under "CURRENT_USER\Software\Microsoft\Internet Explorer\Desktop\General\WallpaperSource". More information about how that works can be found at Is there a windows registry entry for the original background location?.
Getting the username can be done with getpass.getuser().
I don't how customizable the path to the internal copy of the wallpaper is. There is probably an environment variable for the root of it, then you can just append on the wallpaper specific stuff.