Variable: icon-map-list

A list of alists that map icon file names to stock/named icons.
The alists are searched in the order they appear. The first match is used.
The keys in the alists are file names without extension and with two directory
components. For example, to map /usr/share/emacs/22.1.1/etc/images/open.xpm
to stock item gtk-open, use:

("etc/images/open" . "gtk-open")

Themes also have named icons. To map to one of those, use n: before the name:

("etc/images/diropen" . "n:system-file-manager")

The list elements are either the symbol name for the alist or the
alist itself.
If you don't want stock icons, set the variable to nil.