EnableExplicit

Define bgimage
Define hBrush

bgimage = LoadImage(#PB_Any,"C:\...\Dein_Fensterhintergrundbild.bmp")

If OpenWindow(0,0,0,500,250,"Window",#PB_Window_SystemMenu|#PB_Window_ScreenCentered)
  
  hBrush = CreatePatternBrush_(ImageID(bgimage))
  SetClassLong_(WindowID(0), #GCL_HBRBACKGROUND, hBrush)
  InvalidateRect_(WindowID(0), #Null, #True)
  
Repeat : Until WaitWindowEvent() = #PB_Event_CloseWindow
EndIf