Facebook is testing out a new feature that appears in the sidebar of the albums and photo feature called Related Photos. It will display pictures from people that uploaded the album that you are looking at and additional pictures of your friends who are tagged in the album you are viewing according to InsideFacebook.
What makes this feature interesting is that photos by the same author will be displayed despite whether they are your friend or not. This could help increase user engagement on the website. [Inside Facebook]