You can change the logo of your theme by setting up the url in the theme options page. Note that depending on the theme, the logo dimensions is very specific – so you should either convert your logo to the right size, or you’ll have to edit the theme css to fit the logo in the allotted space.
In the admin section, upload your image using the media uploader “Media” > “Add New”.
Still in the media uploader window, look for the field called “File URL” and copy this value:
Now go back to “Appearance” > “Theme Options” (The word theme is replaced by the theme name).
Click “Save” and view your changes.