How to give a unit a custom GUI icon...

[Argh]

Stupid question... how do I define a custom icon for a specific unit? I have a unit that needs to call Activate() in COB for certain things to take place, but I want the user to see an icon called "Deploy" instead of "On". How do I do this?

Sorry in advance if this seems like a dumb question, but I really haven't messed with any of this LUA stuff yet, and this seems like a good way to start getting my feet wet