step 0, check cpan,  [cpan://wallpaper] wallpaper -> Win32::Wallpaper - Modify Win32 Wallpaper

ppm install Win32::Wallpaper
or cpan Win32::Wallpaper
or cpanp -i Win32::Wallpaper --force

then <c> use Win32::Wallpaper; wallpaper('newimage.type', 'tile');

If the above doesn't work on windows 7, ask/google microsoft/msdn