How use bgimage in MUI

I have written these code:
"!define MUI_CUSTOMFUNCTION_ONGUIINIT myGuiInit

Function myGUIInit
# the plugins dir is automatically deleted when the installer exits
InitPluginsDir
File /oname=$PLUGINSDIR\bgimage.bmp "D:\Release_soft\KIDS8.2B\resource\back.bmp"
BgImage::Init /NOUNLOAD /FILLSCREEN $PLUGINSDIR\bgimage.bmp
FunctionEnd"
But bgimage.bmp is not showed!
Thanks a lot!

use GUIINIT not ONGUIINIT 😉
and
BGImage::SetImage
(if remeber correctly)

It's SetBg in the latest CVS version.

Very thanks Richie M and kichik,your answers is very correctly!
thanks a lot!