All of the following icons are not part of Emacs, but distributed and
used by Emacs.

* The following icons are from GTK+ 2.x:

    close.xpm copy.xpm cut.xpm find-replace.xpm help.xpm home.xpm
    index.xpm jump-to.xpm left-arrow.xpm new.xpm open.xpm paste.xpm
    preferences.xpm print.xpm refresh.xpm right-arrow.xpm save.xpm
    saveas.xpm search.xpm sort-ascending.xpm sort-descending.xpm
    spell.xpm undo.xpm up-arrow.xpm

    back-arrow.xpm and fwd-arrow.xpm are slightly modified undo and redo.

    diropen.xpm is file-manager.png from Gnome hicolor theme.

* The following icons are from GNOME 2.x:

    attach.xpm connect.xpm contact.xpm data-save.xpm delete.xpm
    describe.xpm disconnect.xpm exit.xpm gnus/toggle-subscription.xpm
    lock-broken.xpm lock-ok.xpm lock.xpm mail/compose.xpm
    mail/copy.xpm mail/flag-for-followup.xpm mail/forward.xpm
    mail/inbox.xpm mail/move.xpm mail/not-spam.xpm mail/outbox.xpm
    mail/preview.xpm mail/reply-all.xpm mail/reply.xpm
    mail/save-draft.xpm mail/save.xpm mail/send.xpm mail/spam.xpm
    next-page.xpm refresh.xpm separator.xpm sort-ascending.xpm
    sort-column-ascending.xpm sort-criteria.xpm sort-descending.xpm
    sort-row-ascending.xpm zoom-in.xpm zoom-out.xpm 

* Note that the default GTK icons were not overridden by the GNOME theme
due to a bug which was fixed in GNOME 2.15.  Once GNOME 2.16 is in wide
circulation, then the GTK icons should be replaced with the equivalent
GNOME icons.

* Recipe for Creating PBM Versions

1. Edit .xpm image in GIMP.
2. Image > Mode > Indexed. Check Use Black/White Palette and No
   Color Dithering.
3. File > Save As file.xbm.
4. Run xbmtopbm < file.xbm > file.pbm.

Thanks to jan.h.d@swipnet.se for the help.