It depends on the context. "These are hers" is correct if you are referring to photos in your hand and you are talking about those specific photos. "They are hers" is correct if you are telling someone that the photos belong to her.

Hm...What an intriguing multiple choice question. Since the previous sentence is "Look at these photos", I'll choose "They are hers". Here is a similar example: "Look at this photo" -> you will likely to say "It is hers" after that instead of "this is hers"