For some unknown reason one is not able to do stretch without installing an addon. Now you can do it without an addon.

To specify wallpaper to use, has to be a bmp with a 8.3 path format (REG_SZ):

[HKEY_CURRENT_USER \Control Panel \Desktop]
Wallpaper = "c:\image\pamela.bmp"

To specify tiling of the wallpaper (REG_SZ):

[HKEY_CURRENT_USER \Control Panel \Desktop]
TileWallpaper = "1" (0 for non-tile)

To specify stretching of the wallpaper (REG_SZ):

[HKEY_CURRENT_USER \Control Panel \Desktop]
WallpaperStyle = "2" (0 for non-Stretch)

To specify start location of centered (Non-Tiled and Non-Stretched) wallpaper (REG_SZ):

[HKEY_CURRENT_USER \Control Panel \Desktop]
WallPaperOriginX = "1" (Pixels)
WallPaperOriginX = "2" (Pixels)

More Info MS KB135599
More Info MS KB257665