Also there is more than one logo. In fact chances are there are three that you would be concerned with. Here are pseudo directions to replace all three of them (1 client side, 2 staff side).
For clients (ie not authenticated) you would want to replace
OSTICKETDIR/images/logo2.jpg
Or edit
OSTICKETDIR/client/header.inc.php
circa line 15
and change "logo2.jpg"
For authenticated staff you need to make alterations to the /scp side.
either replace
OSTICKETDIR/scp/images/ostlogo.jpg
or edit
OSTICKETDIR/include/staff/header.inc.php
circa line 33
and change "ostlogo.jpg"
AND to fix the login screen
Either replace
OSTICKETDIR/scp/images/logo-support.gif
or edit
OSTICKETDIR/scp/css/login.css
circa line 16 (in the h1#logo section)
and change "logo-support.gif"