I've not used Blogger in a long time...You'll have to edit your main template, I do know that. Once you're logged in and have your Dashboard, click the Template tab. You'll have to have your graphic hosted somewhere online and put in a link to the graphic in your template. I'm terrible with tables and just know basic HTML.
They have some template-related Q&A here:
http://help.blogger.com/bin/topic.py?topic=22
Hope that helps!
Kimberley