I'm working with xpmenu, some of the image placements are confusing and only work in a lower position than expected, this list should help for now, I don't know if it will change with the final release.
Main menu & main submenus:
image
subimage
onsubimage
pagecolor (same as current page)
Menu & submenus:
bgimage
overbgimage
Menu item
overimage (same as swap image)
maz