In un modulo: 


Private Declare Function SystemParametersInfo Lib "user32" Alias "SystemParametersInfoA" (ByVal uAction As Long, ByVal uParam As Long, lpvParam As Any, ByVal fuWinIni As Long) As Long 
Private Const SPI_SETDESKWALLPAPER = 20 
Private Const SPIF_UPDATEINIFILE = &H1 
Private Const NOMESFONDO = "Ciao.BMP"


In un bottone: 



Dim PATHSFONDO As String 
PATHSFONDO = App.Path & "" & NOMESFONDO
SystemParametersInfo ByVal SPI_SETDESKWALLPAPER, ByVal 0&, ByVal PATHSFONDO, SPIF_UPDATEINIFILE


P.S.La costante nomesfondo va modificata per l'immagine che volete caricare