I often see some sort of arbitrary legal notice "applied" to Flickr profiles. (Naturally, I can't find one when I want to publish an example!) The notice basically says "hands off my pictures!" and people seem to think it's legally enforceable. Likewise, there's a similar meme going around Facebook; a draconian-reading "legal" notice that's supposed to be override FB's Terms of Service and be enforceable.
And if you're wondering why - here's the explanation.
In short: the terms of service govern usage, along with the copyright you apply to each picture. For instance, the default copyright on my pictures is "All Rights Reserved". (Yeah, yeah - I know. I can't afford to "go pro" right now.) (Likewise, all the content on this blog is "Copyright Carolyn Ann Grant" - it's the default for Blogger.) Theoretically I could go after someone for using my pictures or text without my permission. In practice - not so much. Still, it's something! But if Flickr altered the copyrights they offered, I couldn't decide to arbitrarily change that to something more to my liking. By using the service, you agree to the terms of service. Those terms, especially when the service is free, almost always include a provision for the service provider to use your data in pretty much any way they see fit.
A more powerful enforcement mechanism than a sternly worded bit of blather is peer pressure. Highlight that you've not given someone permission to use a picture or post you've marked as "All Rights Reserved" and if the community you're in cares - the infringer will embarrassed and won't do it. Mind you, half-way decent folk don't pinch content in the first place, so the fact that someone does, and it's usually explained with a squeaked excuse, tells you all you much of what you need to know about that person. There's fair use, there's parody and then there's being silly. Don't be silly. Don't be "that" guy or gal. And don't rely on silly, stern and unenforceable warnings. As always - don't entrust secrets to the Internet. (And don't do anything you'd rather not see on the front page of the NY Times.)