If they ask "what do you think about me?" and then go all "I'm soooo uglyyy" then yes.


But otherwise,I wouldn't know.

Some people aren't insecure and aren't afraid of letting people see what they look like.
Some people get curious and ask to see what a person looks like.
Some people aren't ashamed
Some peopel know how to practice internet safety and know how to post a picture without getting stalked

It depends. As alismouha said, those kinds of people are obviously just fishing for compliments. The same goes for posers who claim to be these scene kids, when they really found the pictures on the internet. However, some people aren't like that. Some post pictures of themselves because they're simply comfortable with their looks, or they want to be honest, or something like that. *shrugs* The motivation for posting pictures varies person to person.

Not all, of course.
But a lot of people, especially teenage females are taking either: rather provocative or slutty pictures of themselves, or photoshopped pictures, or putting pictures of models saying it's them.
Summed up, they are obviously either insecure or looking for attention.
There is also the well-known "guido" case, where a notable number of males is lurking on the chat, their profiles filled with pictures of them, trying to look "cool" and adding every freaking girl on the site.

And of course, they're all "lol im not pritty at all hunni but thx!!! xxx"

That depends on the person to be honest. If they ask what you think and complain about themselves in the same sentence then they most probably are. However if a person posts a picture and then asks does my new haircut suit me do you think? Then they most probably are only looking for security on how they look as the may be a little wary of their new style.
But i see nothing wrong in giving an opinion when someone asks how they look, so long as they can take the truth. I can always find something good about every person (usually) but if you're ugly, i will tell you you are ugly no matter what the consequences. I'd rather be honest and be called every curse under the sun for my bluntness, then lie to someone.