Michael, the easiest way is to use a theme that's already set up for a logo, for example. the "standard logo" theme. If you use that one, all you have to do is save your graphic as logo.jpg and put it in your main Moodle folder.
The more complicated way is to edit the header.html file for your chosen theme and insert the HTML and/or PHP code necessary to display your logo in the appropriate place. Here again, looking at the header.html file in the standard logo theme would be useful to learn how it works.
I've gathered a lot of the theme code is going to be changing in 1.5, though, so maybe spending a lot of time on theme tweaking wouldn't be wise right now.
The more complicated way is to edit the header.html file for your chosen theme and insert the HTML and/or PHP code necessary to display your logo in the appropriate place. Here again, looking at the header.html file in the standard logo theme would be useful to learn how it works.
I've gathered a lot of the theme code is going to be changing in 1.5, though, so maybe spending a lot of time on theme tweaking wouldn't be wise right now.