Not just photos, but in general I've found that the people who are continuously posting how happy their lives are and how everything is great are most likely the ones who are utterly miserable. I have been seeing a woman's posts lately who is going through a bitter, ugly divorce, and it's astounding how she represents everything publicly via Facebook when I know the full story due to a family member being involved.
This research is for finding a clinical condition and make less expendable the cost of health programs, not to evaluate the life of others based just on opinions. Suicides are the second-leading cause of death among teenagers. But, some people are just "not a happy person". That doesn't mean that they are unhappy, sad or suffering from depression.
I really like this comics from Oatmeal that talk about that. http://theoatmeal.com/comics/unhappy the essay that this comic is based thttps://outline.com/r6q4Ga