I posted a response to this on another BB, but will re-post it here because I do think it’s relevant. I will also add that if you can get the stallion owner to write the letter, even better 
Ugh…How unfortunate. I know I’ve got literally 100’s of photos in my files on offspring by our stallions. I TRY to keep track of who owns them and who took the photo, etc. But I know that occasionally I drop the ball.
Under the circumstances, I’d be a bit irritated, as well. BUT, you can still turn it into a positive situation. First, I “would” send a polite letter to the SO saying that you have no issue with photos being used of your horses, but that you would really appreciate credit for the photo and acknowledgment of you as the breeder of the foal and unfortunately that was not done so. Also say that obviously, you’re an advocate of the stallion as you bred your mare to him, so it would behoove said SO to include you in any stallion endorsements for obvious reasons. NOT including you leaves a bad taste and the potential of bad press for said SO.
Next, if you have a nice updated photo of the foal/horse in question, take that and send it to the publication, along with a letter to the editor saying something NICE, not snarky, such as "The ad in the stallion issue on page XX for “Casanova included a picture of our foal, Junior. Due to an unfortunate oversight, we did not receive the photo credit. We’d like to correct that and here is an updated photo of Junior”. LOVE your magazine, blah, blah, blah. You not only get people to go back to that ad and see the picture you are referencing, but you also get double duty by putting in a new photo of your kid Win/Win. Bad situation becomes good! Stallion owner receives a polite reminder to not tick off clients and/or potential clients. Puts them on notice. And if you can get the stallion owner to write the letter, even better! They probably “should” do it anyway as a matter of courtesy and a great way to apologize for the oversight!
Originally Posted by JBandRio View Post
Send a certified letter written by your lawyer that you will take legal actions if she doesn’t comply. That’s often enough to get action. But, be prepared to back up your threat if she still ignores you.
(this is a quote from the other BB and not from COTH)
Absolutely one of the worst things you can do. Truly. You become considered an unpleasant person to deal with and your reputation suffers for it. Think about it. Threatening legal action over a misappropriated photo? Not cool. I “would” make sure that I sent a polite letter as outlined above. But, I think most people are flattered if a photo is used of their foals. The big problem is not receiving credit for it. And I can certainly understand people being upset about that.
Heck, we have articles and information taken all the time off of our website. We will contact the “guilty” part and ask for appropriate credit and links be included. If at that point it’s not done, we will ratchet it up, but those articles and information are our professional reputation. And, part of the reason we request a link back is because things don’t remain static and it’s a way for readers to verify that the information is accurate and current. But photos are used regularly and we don’t get too uptight about that. …
I hope the above helps!