The Rate & Review panel in the Loupe view allows you to quickly give a star rating and flag (pick or reject) your album photos.
The Filmstrip at the bottom allows you to cycle through the album photos.

(Left) Rating a photo. (Right) Flagging a photo in Rate and Review panel.
Rate a photo
To quickly rate a photo in the Rate & Review panel, flick upward or downward in left-half of the screen. A rating overlay appears showing you the current rating as you flick.
Alternatively, tap the Star rating graphic (just above the Filmstrip) to bring up the rating overlay and then choose a rating.
Hope that helps.
~Akash