Usually if you have one frame and you try making it completely transparent, it won't let you, saying that it must have at least one non-transparent pixel. But if you make two or more, and the first one is fully transparent, then it lets you. I did this (stumbled upon by accident) and I don't know of any way I can go back and change it, Undo won't work and I can't double or right click it. There's supposed to be an option to Edit the image somewhere, right?

Ah, found it in the workspace toolbar... still its odd there isn't an "Edit image" option in the "Edit" menu...