It doesn't show up in the properties window for either control, and equally no ImageIndex property for tool buttons or menu items.
You can assign manually to the properties in code, and Intellisense happily suggests that they exist, but then tool bar images work and menu images don't.
Hey ho. Never mind. Found a question about it on the MS forums but no-one had answered it. I appreciate that this is free software and am not complaining, but I wish that they would at least document what they have and haven't disabled.
Time to give up on C# for a bit anyway as I want to start my next game.