Once enabled, a new "feature this" button will appear next to the other image control buttons (delete, rotate, etc). Clicking it will set the image as the site's current feature, which will be shown in the side bar of the post list.
Viewing a featured image
Downloading a featured image
/featured_image/download. This will give
the raw data for an image (no HTML). This is useful so that you
can set your desktop wallpaper to be the download URL, refreshed
every couple of hours.