Starting with version 0.5 you can upload your logo as either logo.gif, logo.jpg, or logo.png to the /images folder and it will be used in place of the <!--logo--> and <!--logo_link--> variables.  The basic white theme, again starting with version 0.5, uses this variable in the header.

In version 1.0 you can upload files directly to your theme folder and edit the theme file to display your graphic.