I created tango-based transport icons for gajim, so if you'd like to have your roster display icons that match the new tango icons, you can download the tar.bz2 file from trac on gajim.org.
(since this is a replacement procedure, you may want to backup these directories before overwriting them with these new icons.)
simply extract the three directories (aim, icq, and msn) in the tarball to /usr/share/gajim/data/iconsets/transports, writing over the aim, icq, and msn directories that are already there. (you may need to be root or use sudo.)
Restart gajim and the new icons will be in the roster window.
Here is a sample screenshot: