Don't make it too wordy. Get to the point: Gender, age, training, show history, if any.
I avoid any ads that say, "Experienced Rider," bc it tells me that the horse has serious training problems, and the owner is selling bc they can't fix them.
Also, put your horse in front of something that looks nice.
If I was selling one of my horses, I would take photos in my South Pasture, where the previous owner planted 40 evegreens, now about 20-30 ft. Tall, and the grass looks lush. I would avoid the north side of my garage where I need to clean the green moss off of the white paneling. Some sales photos are in stalls with cobwebs and chewed wood, and it turns the buyer off.