What's the significance of the photo shoot in the opening credits?

I think it is to establish a basis for what the characters were like/ who they were before the outbreak and the transformations they've gone through before the start of the movie.

Since a common idea in disaster movies is about how people change in response to an adverse change in environment, I think that this movie was trying to reinforce that the characters were once 'normal' and we are viewing a transformed version of them, with these photos showing the 'before'. I think the fact that the present version of these characters are holding the photos reinforces this comparison.