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

Public Const SPI_SETDESKWALLPAPER = 20

Dim lngResult As Long
Dim strBmp As String

strBmp = "c:\yourimage.bmp"
lngResult = SystemParametersInfo(SPI_SETDESKWALLPAPER, 0, strBitmapImage, 0)