Thanks guys, I will definitely get better pictures next time I go out to the barn. She does have a very short back, so the saddle takes up pretty much all of it, which might be contributing to the problem. But it's definitely sliding forward, not back so I don't think a breast plate is going to do the trick. Plus that saddle doesn't have D rings, so it's hard to get one properly adjusted anyway :(
It is pretty snug through her shoulders. She doesn't look it, but she's really broad; I have the extra wide gullet installed on my dressage saddle and it fits her beautifully. I was hoping I could make this one work, but if it's not fitting right I'm probably better off shopping for a new one.
Also, I think part of the slipping problem has to do with her conformation. I've noticed she even tends to pull me forward when I'm riding bareback.