Superhero does not have a full width header image. The full width images are in the featured post slider.
> Appearance > Header
Suggested width for the header image is 480 pixels. Suggested height is 150 pixels. http://theme.wordpress.com/themes/superhero/
If you want to have featured images in posts appearing in the featured post slider across the top of the blog then you have to set featured images. Have you read the theme description at "Featured Content Section" here? http://theme.wordpress.com/themes/superhero/
You need to Create a post with a featured image that’s at least 960px wide, set it as a featured image and give the featured post the tag you declared under Settings → Reading → Featured Content.
If you don’t see the Featured Image module on your Add New or Edit Post page, please make sure you’ve selected Featured Image in your Screen Options. If you don’t see the Featured Image module on your Add New or Edit Post page, please make sure you’ve selected Featured Image in your Screen Options.