I've tried using S06 portraits (Sirius Chill's Horse Portrait Mod:
http://www.startersorders.com/phpBB2/vi ... 21&t=10885) for S07 and haven't had any luck, the game just has empty white boxes where the pictures should be. I'm working on my own custom horse portraits and would like to know how to make them work, but currently with the fiddling I've done, I cannot seem to get them to work just yet.
Edit: So I've found that if I put a portrait of the horse image I want and re-name them a horse in my current game the photo is replaced with it.
I hope there is a work-around (I'm not a modder) instead of having to hand-name every horse icon so it shows up on your horse(s) in game.