It's an internal feature in Mosaic for X, to get cute icons in Gopher
menus (and now FTP menus). We have not documented it nor intended it
for general use, but some people (USGS) have been using it in
generally available documents anyway... sigh...
The bitmaps themselves are in the Mosaic source code (src/bitmaps), in
xbm format.
Cheers,
Marc