This is a theme/iconset for Gajim.
It was design to go with the full Gnome/XFCE icone theme : http://gnome-look.org/content/show.php?content=81153
Open the archive, go in the Gajim base directory (on Ubuntu : /usr/share/gajim . If you have trouble to find it, try typing "locate gajim"), then in /data/iconset, dans copy the Buuf folder inside iconset.
You can now choose Buuf icons in the Preferences menu.
If you want to add more Buuf icon to the interface, you can replace the icons in data/pixmaps with those included in the archive, but keep in mind that changing files used by Gajim is not a benign operation, so DO IT AT YOUR OWN RISK.
All credit for these wonderful icons goes to Mattahan : http://mattahan.deviantart.com !