This is what I have on Fresh and Clean in my notes:
Fresh and clean theme supports header of 920 X 116 pixels
Make it sure your header format is one of these
The post slider appears on the front page if and only if you set featured images of the size required by the theme and mark them as sticky posts.
Featured Images at least 840 pixels wide.
(optimum width is 290 pixels for featured images)
Featured Images as Post Thumbnails
If you add a Featured Image to non-sticky posts, or if you add a small Featured Image to a Sticky Post, that image will appear as a prominent post thumbnail beside the post on the front page, archive pages, and in search results.
Fresh & Clean
Sticky Posts Featured Slider: 840 x 280
Thumbnail: 160 x 120
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. http://en.support.wordpress.com/screen-options