make these changes:
in your theme's header file (most likely header.php)
Find this line:
add this as a new line beneath it
In your CSS stylesheet
find this section
margin: 80px 0;
add as a new line between the brackets
add this as a new section at the bottom of the sheet
background: url(images/logo_filename.jpg) no-repeat;
Change filename to the name of your logo file - put the file in your theme's images folder.
Change height and width to those of your logo image.
As you view the site, adjust Left and Top to position the logo where you want it in relation to the header text.