Both KDE and GNOME use PNGs natively (also optionally SVG), and all icons are in the folder /usr/share/icons/[your current theme].
You can replace the icons you find in that folder with PNGs. or make your own theme.
I got this information from
http://baghira.sourceforge.net/icons.php.
Also see
http://standards.freedesktop.org/ico...t/ar01s07.html.
For prepackaged themes/icons, go to gnome-look.org, art.gnome.org, baghira.sourceforge.net, etc.
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