I always understood that the objection was to posting photos of other people's horses FOR CRITIQUE (including "should I buy" or "should I breed to" questions)
There is nothing wrong with posting a previously published (including the internet) photo of a horse simply for reference. The owner has essentially placed the image in the public domain, and it is hard to object to the publicity an approved photo will bring. Of course, it is essential to credit the photographer and owner if at all possible. (If this were wrong, most farm websites would need to delete all of their reference sire photos).
On the other hand, I would never post a photo that I had taken of someone else's horse without their permission, especially if they are less than optimally presented.