Marble, for sure(:
Negatives: Short in neck, steep in croup, low set tail, and she may be downhill, but I think it's just the picture I'm looking at
Positives: Good structure in legs, good angle to pasterns, good build in shoulder, strong back
When you judge a horse, you're looking at three major things: Balance, Muscle, Structure. So I'd recommend getting more muscle on her, especially in her hindquarters and across her topline, as well as working some of the chub off! Along with BSM, a judge looks for the way the horse moves. Choppy, rough movers will place below fluid, pretty movers.
Also, you don't want her to have any attitude. She should be quiet and well behaved –*enthusiastic, even! Teach her to set, too. Practice this at home a lot. Especially when horses get to new places, they can be antsy and won't set well – this will definitely get points docked!
When preparing her for a halter class, make sure her mane is clean, trimmed, and neat. No long, ratty manes! Her face, legs, and neck should be trimmed –*no long whiskers or fuzzy hairs! As well as a short bridlepath.
Hope this helps – let us know how it goes!!