What follows applies only to the free hosted WordPress.COM version of the Superhero theme. Note that ins order for featured post sliders to operate one must adhere strictly to the exact dimensions of the featured images required by the theme.
Please consult the theme description here. http://theme.wordpress.com/themes/superhero/
Superhero features a Custom Header that can be set under Appearance → Header. However, many bloggers assume the featured Featured Content Section at the top is the header when it is not. You are confused. Superhero does not have a full width header image. The full width images are in the featured post slider.
Superhero header > Dashboard > 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.