DOCBOOK-APPS: Graphics for navigation links

I notice that graphics for 'Home', 'Up', etc. navigation links were included
in the distribution of the Docbook modular stylesheets. I can't find a hint
as to how to "turn them on."

I have grepped though the Docbook modular stylesheets, and tried searching
deja and the mulberry dsssl list, but to no avail. 

My recollection is that the Cygnus apparatus (db2html and associated
stylesheets) defaults to using graphical nav links, but I'm not sure this is
helpful in my case, since I have dsssl driver file well underway for the
modular stylesheets.

Also, has the Unisys unpleasantness cast a shadow over these gif files, and
if so are there some png replacements out there?

Thanks for any light that can be shed on these arcane mysteries! <g>