Ok, basically this is impossible to do using any of the regular menuing stuff. I started poking at a program that does this, claws-mail, and found the answers to this in file 'trayicon.c' for it's tray icon plugin. You can find it all in the functions 'create_trayicon' and 'click_cb'. Basically what needs to happen you need to create a event box with the image in it. Then on a mouse click, you then activate the menu.
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