There's nothing wrong with asserting copyright on your photo. I normally send out a message or e-mail to the offender so they can make me whole again before I take a more stern approach to asserting copyright. FB will respect your copyright and will promptly remove the photo from the system. I specifically provide watermarked images to B&G for this purpose. Normally, everyone respects the copyright because I have it stated in bold in my contract as well.
She sent one back asking if I could be more specific, and that she can't see where she's gone wrong. So she doesn't know how, or can't even remember where she's used the photos.
That's probably true -- she's probably displaying dozens of images in multiple locations, and may have no idea which ones are yours.
You could go to the trouble of sending her the watermarked images again, and asking her to look at each display location for matches from that set. But you might just decide this is not really doing you any measurable injury and shrug it off.
It's unlike a competing photographer claiming your work. And I think Todd is right: the volume of referral business you'll get from a MUA is likely zero, both because she probably has low traffic to those sites, and also because I'd bet most brides book photographer before MUA.
Even SEO-juice benefits are likely low. Not seeing a lot of reason to pursue it. (And I'm an IP attorney who almost always sees a reason to pursue it, at least in principle.)
I'm not with the other replies to let it go. It doesn't take much to set her straight by sending another more specific reply. Protect your work! Not at all cost, but do protect it.
As noted, she may actually not know which images or how to not crop them; I say this because I have found trouble properly uploading images to FB. Matter of fact I only use it because for business sake I have to. Everything about FB is difficult to use....and I consider myself tech savvy.
Last resort is to tag/report the images to FB people for removal.
Getting upset that someone shared your photo, and is using it outside its intended purpose, is a natural feeling. Sure we all want to make the most out of our images, but I think this is something that we photographers just need to understand: it's not going to stop nor change anytime soon.
Imagine walking into a burger joint with a friend. You order the burger, your friend orders nothing. You share a bite of your delicious burger with your friend, and then you both get kicked out by the manager because you shared the burger.
While image copyrights are clearly different, the restaurant guests will feel ostracized regardless of whether it was permissible or not. Be good to your clients, make friends with vendors, it'll pay off a LOT more than chasing down copyright infringers.
