So creating an icon means drawing an image in several sizes and color depths. Vista recommends that all icon have a 256x256 @ 32-bit page, for which all other pages all derived. Maybe the most important image formats are the following:
Note that the color depth cannot be specified explicitly in GFIE, it is updated dynamically according to the picture contents. For example, if you want to create a page in 256 colors, draw an image which has at most 256 colors and does not have semi-transparent pixels.
The newly created icon may have pages with the same size and color depth. For example, there is no such format as 16x16 @ 24-bit, since that page would have 256 pixels and that means at most 256 different colors - so the format is rather 16x16 @ 256 colors. In this case, you should remove the duplicate pages to avoid redundancy.
See this article.