Navigate to Administration>Manage Albums and select a Category then an Album. Each image in the Album will have an 'image' icon in the lower right corner.
Tap the image icon on the image you want to be the hero and then tap OK in the confirmation dialog box.
Tip: If you want an image to be the hero image without being part of the album search results: Change the item to Access - Item Group to Administration, this way only Administration Access Group can see it. This is a handy option for when using illustrations or logos as the hero image for an album.