Bob Sullivan writes a great post about the pros and cons of sharing photos on social sites such as TwitPic and others. The rights the photo takers have and the rights that journalists and the like have. This is a very interesting problem that we find ourselves in right now.
In the past I have always stayed cautious, not sharing something without making sure that it reasonably cannot be stolen (locking down Flickr settings, making copyrights clear). One great piece of advice in the post from attorney Carolyn Wright offers:
If you don’t share your work online, then no one knows to license it.