editing icons

I'm using Visual Studio 2010 and I'm having problems editing icons.

It's an MDI app, and the icons I'm trying to edit are located in Resources->project.rc->Icon->IDR_MAINFRAME

Previously they were the default "MFC" icon with 3 little blocks. I went to edit them, and got about two thirds down the list of icons (editing them to look like how I wanted), but in the last third the bitmap editing tools (pencil, brush, fill, etc) are all greyed out and I cannot paste into the icon either.

Is this a bug, or do I need to go do something somewhere to let me edit them?