Significance of the Laura photograph?

Laura is understandably surprised to see the photo because she presumably recalls throwing it away. The main reason people throw photos away (apart from technical flaws) is that they haven't taken a good photo, which is backed up by the inspector's comment that she doesn't look much like her photo. You hear it all the time, people don't think they look like their photos because they are used to seeing themselves in mirrors which inverts the symmetry of the face. You have to take the scene in context with the other scene where the inspector views the sketches of the sisters, looks out of his window and sees them but fails to make them. This is really the whole conceit of the film: people see things that are not there, and people don't see things that are. That's my interpretation anyway.

Also, when and where was this photo taken place? That could help solve some of the questions as well. I thought perhaps it was somehow taken in the house with the sisters...she seemed to be wearing the same clothes...I will have to go back and look more closely.