Private Declare Function SystemParametersInfo Lib "user32" Alias "SystemParametersInfoA" (ByVal uAction As Long, ByVal uParam As Long, ByVal lpvParam As Any, ByVal fuWinIni As Long) As Long

Private Const SPI_SETDESKWALLPAPER = 20

Private Sub Command1_Click()
Dim lngSuccess As Long
Dim strBitmapImage As String
CommonDialog1.ShowOpen
strBitmapImage = CommonDialog1.FileName
lngSuccess = SystemParametersInfo(SPI_SETDESKWALLPAPER, 0, strBitmapImage, 0)
End Sub

This is the simplest way to set the wallpaper .....