My layman's understanding (based on consultation with IP lawyers and personal experience) is that almost anything that anyone creates is copyrighted as of the moment of creation. Enforcing the copyright, however, requires additional steps, including valid copyright notices and registration of the copyright.
Actually collecting damages also requires a finding that a reasonable person coming across that content would know or have reason to know that the content was copyrighted, and used it anyway. If the source from which they obtained the content represented it as being in the public domain or usable under a public license (Creative Commons, for example); or if the secondary source misrepresented themselves to the alleged infringer as owning the copyright and granting permission; or if the image was made available under terms that were too vague; then all the legitimate owner is likely to collect is the cost of a license to use a substantially similar photo from a stock photo site.
One company has made a cottage industry of threatening to sue people who used copyrighted photos that have been distributed far and wide on the Interwebs. In almost all cases, the alleged infringers had reason to believe they were using the pictures legally, either because the pictures were represented as being in the public domain; because the photographers had listed the pictures on other stock sites and simply had forgotten; or occasionally because the alleged infringer purchased "rights" to the pictures from entities who did not themselves own those rights.
In any of those cases, the most the actual copyright owner or agent could reasonably expect to collect were push to come to shove would be the rights value of a substantially similar stock photo. If the rights to the picture were purchased from an agency that once handled rights to that picture, even if they no longer do, then the photographer can collect nothing.
I've been on both sides of this battle. I've had so many stock pictures infringed that I don't even bother sending out nastygrams anymore. I just assume that anything that goes on the Web will eventually be infringed. It's not worth the hassle and cost of litigation for the few dollars I might collect. If the alleged (or even actual) infringer pulls up a substantially similar picture with a license fee of $6.37 on some microstock site, getting anything more than $6.37 out of them is exceedingly unlikely barring intentionality of the most grotesque sort.
I've also been threatened with lawsuits by stock companies for pictures that I legitimately bought from other stock companies. Lots of times photographers simply forget that they licensed the picture to multiple companies. Other times they lie about it when the company that pays best wants exclusive rights to the picture (for example, for a book cover illustration). The photographers just pull the images from the other companies and hope that no one who already bought rights is targeted by the company that now has the exclusive (which would severely damage or perhaps even terminate their relationship with that agency once the target proves that the rights were purchased through another agency).
What it comes down to is that you own the copyright: but your chances of collecting anything beyond what a substantially similar photo would command on the stock photo market are basically zilch. And that's assuming that your publication of it included copyright notices and that you registered the copyright. Infringement cases are almost never actionable at all if those two requirements haven't been met.
Again, I'm not a lawyer. I'm just a person who has been through both sides of copyright battles for the past 20 or so years. None of them have resulted in my getting anything more than an apology and a Starbucks gift card or something along those lines. Neither have I ever paid so much as a penny for infringement alleged against me because I have receipts for every picture for which I ever purchased usage rights.
In summary, alleging infringement is easy. Proving infringement is also easy in most cases. Collecting on infringement, not so much.
Rich
