I'm sure there is an in-depth explanation for them, but let me just throw one instance out there. When I was young, I knew a couple of girls who were into seeing how many fish they could have on the line at one time - and the time happened to be when their birthday was approaching. Just saying.
Most of them have low self-esteem and want someone to say things like they are sexy or pretty to make them feel good. Pictures can make a person look 100% better than what they do in person and a lot of the pictures could be of what they used to look like.They also like to boost confidence when people say they look good.It gives them a sense of 'power or dominance' beside fishing for more friend adds.