Private Declare Function SystemParametersInfo Lib "user32" 
Alias "SystemParametersInfoA" (ByVal uAction As Long, ByVal uParam 
As Long, ByVal lpvParam As String, ByVal fuWinIni As Long) As Long
Const SPIF_UPDATEINIFILE = &H1
Const SPI_SETDESKWALLPAPER = 20
Const SPIF_SENDWININICHANGE = &H2
Private Sub RemoveWallPaper_Click()
    Dim X As Long
    X = SystemParametersInfo(SPI_SETDESKWALLPAPER, 0&, "(None)", 
SPIF_UPDATEINIFILE Or SPIF_SENDWININICHANGE)
End Sub
Private Sub ChangeWallPaper_Click()
    Dim FileName As String
    Dim X As Long
    FileName = "c:\yourBMP_file"
    X = SystemParametersInfo(SPI_SETDESKWALLPAPER, 0&, FileName, 
SPIF_UPDATEINIFILE Or SPIF_SENDWININICHANGE)
End Sub