I am not an attorney so I can't give legal advice.
I am not sure people are aware of this unless they read all the terms and conditions of all the sites they join. By becoming a member of a site like Flicker for one you grant a release to the site to post, edit, and put your images in their advertisements. This is the exchange you do with the website for allowing you to post for free. I have the feeling that many people may not be aware of this. If you see your image in an advertisement for the website, posted somewhere else with reference to the website, and/or even if you notice an edit on the image you may have granted the website the right to do so in the terms and conditions. By becoming a member of the site you have agreed to their terms and conditions.
Of course I am not an attorney so I can't give legal advice.