You can favorite any of your photographs or screenshots in the stock Photos app on your iPhone to help filter your most-loved memories from the rest. In order to favorite a picture, you normally tap on the heart-shaped button at the bottom of the app.
Heart Throb, on the other hand, is a new free jailbreak tweak that brings Instagram-like favoriting capability to the stock Photos app. With it, you can double-tap on your photograph to “favorite” it.