Please see here for the description of how the theme is designed http://theme.wordpress.com/themes/autofocus/
AutoFocus’ prominent single post featured images, which when set manually are placed front and center, above your post’s title and content. The maximum width of single post featured images is 800.
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.
AutoFocus responds to not only single image posts but also full galleries. If you would prefer to forego a large featured image above your post content and instead showcase an entire set of photos then simply insert a gallery into your post without manually setting a featured image.
Main page and archive pages on Autofocus don't rely on featured images. Featured images have to be set manually to each post. Each post on the main page and on category/archive pages will display a regularly inserted image (provided the image had been uploaded via the Add Media button of that post).
The only difference is when you view a single complete post: regular image shows below the post title, max width 590px, featured image shows above the post title, max width 800px.