This might not be the best way to do this, but if you have access to the theme editor under Appearance, you can change the text within the <body> tag on the header.php page. If you open the Header page, scroll down until you find <body <?php body_class(); ?>>. There you will find the code for the site title bloginfo( 'name' ) and explanation bloginfo( 'description' ). These can be replaced with an image. However, if you use another theme you will lose these changes.
