Is that a question one honestly needs to ask?
The long story:
People generally want a bit of anonymity or privacy. Hence why people go by pseudonyms (most of the time), and that's also likely why they don't really flaunt pictures of themselves or those they care about all over the Internet. You never know which sicko will see it (especially here).
With that said, there's also probably some self-confidence issue that most Internets frequenters have. You know, obese people with self-diagnosed Aspburgers syndrom and a hefty helping of pimples and have probably not bathed in a week. You know, WoW folk.
Pick either/or definition on a case by case basis. It's always one of the two.
Oh, and I guess some people like posting funny pictures or something that they connect with on some level, for whatever reason. I thought Jack Black's flaming microphone from PoD was funny as shit, so there it is.
Meh, I've given this more attention than it deserves.