it really depends on your blog theme/ If you are using WordPress, click Presentation, then click Theme Editor. More than likely you wll need to make a change to either header.php or style.css or both. And you will need to upload your new header graphic into your theme's image folder.
From your dashboard, click Layout. On your layout click 'Edit' for the widget that has your blog's header name. If its the title for your blog, the edit widget window will have an option to upload a pic from your pc or type in an URL of an image from net. Now check' Replace title with image' or so. Now your blog's title will be replaced with the new image....