Here is my feeling about it:
I don't mind when someone wants to use my photos for promoting the horse they bought from me. I do mind if they are using them for sale ads unless it is a recent sale to the purchase from me (as in the pics are current and good examples of what the horse looks like).
I wouldnt' say anything either way, as susanne mentioned, it's probably not worth the time to make a fuss about. I just feel badly for the other shoppers if something has changed drastically about the horse and they're seeing "old" and outdated pics.
Here's my experience:
I owned a cremello mare. One day to get her pics for her reg. papers, we bathed her and cleaned her for about three hours...we live in a very muddy climate and she was just brown from the dust and dirt in her coat. We showsheened her and groomed her and took about 200 digital pics from all angles b/c I was getting them for her perm. A papers also for possible sale.
She was in great condition.
I sold her to a lady that Winter. The pics were on her reg. papers also posted in the ad where I sold her.
Four and six years later, these same photos I took so long ago are used for sales ads by a breeder that does NOT take care of their horses. The mare hasn't grown or changed any as she was five at the time, but when I saw her at this farm in '03, she was a skeleton. The photos I took show a healthy, clean horse w/no problems. I saw her 2 years after I sold her and I can hardly believe she's alive, but I KNOW this mare does NOT look like she did in those photos, and whoever would buy her off of those photos would be in for a rude surprise (this person sells over the internet a lot.) It bothers me a lot, but more for the fact that they are being used for deception. In my case, the pics are part of her reg. papers and so must be the property of the new owner.
I guess what I'm saying is that most sellers would want to provide recent pictures anyway, but if they do use yours, it would be great if they asked permission. Next of all, if all else is fine, just look the other way and shake your head and move on. There's far worse transgressions happening around us.
Liz M.