As discovered in Bug 13651, the option "Show images in menus" under Appearance when disabled has no effect on xfdesktop.
It happens because xfdesktop is not using GtkImageMenuItem (deprecated), instead it follows Gtk's recommendation and packs a image and label into a GtkBox. Therefore it would need to watch for changes of the /xsettings/Gtk/MenuImage property, then add a padding image (gtk_image_new_from_icon_name("", GTK_ICON_SIZE_MENU)) if disabled. I think it's too much trouble, garcon also needs this logic, so I decided to not support that property. Reopen if you disagree, preferably with an attached patch.