On the www.wordpress.com site you will find a plethora of great tutorials and that is where you will also find a link to WordPress TV. I just watched a great little video on how to customize your WordPress.com header (not the self hosted .org version).
In WordPress.com only certain themes allow you to easily change the image that appears in the background of your blog’s header (the part at the top with your title and tagline). This video shows you how to find out which themes allow you to change the header images and then it talks you through changing yours.
