It took me a while but I eventually found out how to add some custom images to the WP 2011 theme.
Here are the links which helped me:

• http://www.wpbeginner.com/wp-themes/how-to-enable-custom-header-images-panel-in-wordpress-3-0/
  
• http://www.blog.web6.org/twenty-eleven-header-image-how-to-change-size/
  
• http://wpti.ps/functions/replace-remove-header-image-twenty-eleven-theme/