I've long been surprised at the dearth of good tools for dealing with Windows icons. For example, considering how mainstream a format it is, I'm surprised to find that neither Photoshop nor Gimp can handle it. Perhaps it's because of .ico's inherent multiple image nature.
In particular, there haven't been any useful freeware icon editors that I've found. Well, now I've found one: @icon sushi It isn't exactly an icon editor, since it focuses on all aspects of icon other than the color of the pixels (in other words, all the things that are unique to icons). For painting pixels, use your favorite image editor, then use @icon sushi to pull the pictures together into an icon.