The button/icons in the EC are saved as '.dds' files, which GIMP can interact with.
To edit/create the icon files, you might want GIMP, the GNU Image Manipulator Program. I see that both Petra and I are using it. Rather than replacing a file, I'm going to post some instructions for how to add custom icons to your macro icon selection.
Like with most things in the EC, there are multiple ways to do the same thing. What TimSt and Petra have worked out above is great way to replace an icon. Since this thread is current and Petra asked a direct question that has a straightforward answer, I'm going to post some steps here (a very skimpy walkthrough, if you will). I've been thinking about doing a walkthrough for creating icons but haven't had the time.
