Taking photos is an interesting thing, isn't it?
I figure that if someone isn't profiting from your work, or degrading your reputation, then it's no big deal. But you can always stick to the letter of the law. It's your choice.
As a teacher, it's not uncommon for me to show students a picture online. For example, if I'm teaching about the Panama Canal, and am trying to explain how locks work, I might simply go to Google, type in "panama canal" and click images. Then I'll pull up the images and show the students (I keep my google on strict filtering, just in case something goofy is in there, like girls in bikinis). Often people let their albums on flickr become public, and those images show up in a search.
If someone takes your photo uses it to make money, or photoshops it to where you would be embarrassed, I think that's bad.
But nine times out of ten I use Wikipedia and it has images that are fair use and contain stipulations for people who plan to use the images in their own work (give credit, not for profit, etc.).
My facebook pic is my own photography/art. Others should do the same. Creating a profile photo isn't difficult, and taking someone else's work is just lame and uncreative.
Thank you. 