ICO files are used to
contain Windows icons. They're very small bitmaps. If you convert a larger
bitmap to ICO, only the upper left corner of the source image will appear in
your icon.
ICO files consist of a
bitmap and a transparency mask. When you view an ICO file with GFX-View, the
transparent colors will be replaced with a solid color – by default it will be
gray.
GFX-View
will read icons of any size and color depth recognized by Microsoft Windows.