A featured photo represents a gallery.
The default for non-password-protected galleries is to rotate randomly between all photos in your gallery. The default for password-protected galleries is to display a "Protected Gallery" graphic.
To pick a feature photo, you must be logged in. From the Tools Menu above the photo you'd like to feature, select Feature.
Later, if you select Remove Featured Photo, random rotation (or the "Protected Gallery" graphic) will return.
That's all there is to it.
What about categories and subcategories?
There is currently no way to designate a feature photo for category or subcategory, sorry! Instead, we'll use a thumbnail from the first gallery displayed inside that category or subcategory.