For Joomla 1.5 (although it should be the same for the newer beta versions of Joomla), you can create your own logo with the dimensions, 284x52, which is what i use.
Then upload that graphic to the \images directory. I use FileZilla client to FTP in to the site in order to upload new graphics etc. (Joomla's FTP settings are in the Global Config)
Next, you should go to Extensions > Template Manager - Once there, select your template and click "edit", and then "Edit CSS"
Then click the radio button on the left of "template.css" and select "edit" and find under the "div#logo " tag where it says
background: url(../images/mw_joomla_logo.png) 0 0 no-repeat;
and simply change the
to the name and file extension of your logo. Then just click the save button
Hope this helps