Theme description http://theme.wordpress.com/themes/mystique/
Featured (“Sticky”) Post
One featured post will show up in the header if the post is set as sticky, has a featured image attached, and the featured image is at least 940 pixels wide. If you have more than one post that meets these criteria the most recent post will be displayed.
The featured post area will show in the header of every site page unless you expressly turn it off in Appearance → Theme Options; in that case it will only be displayed on the home page.