REPOST:Is my breast collar to low on my horse? (picture)?

It looks fine to me, doesn't look like it should restrict her movement in any way. Honestly if it was up higher it more than likely would restrict the movement of her shoulder as well as her neck. As long as it is not overly tight, yet not loose either, there should be no issues with it. However, if you notice any problems while riding or if it seems to slip and slide a lot you could also purchase an over-the-neck breast collar strap to help hold it up a bit and they are adjustable to whatever length is most comfortable for the horse.