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
Const SPI_SETDESKWALLPAPER = 20
Const SPIF_UPDATEINIFILE = &H1
Const SPIF_SENDWININICHANGE = &H2

dim a as Long,strFileName as String
strFileName="c:\windows\desktop\a.bmp"
a = SystemParametersInfo(SPI_SETDESKWALLPAPER, 0, strFileName, SPIF_UPDATEINIFILE Or SPIF_SENDWININICHANGE)