@Conrad_Max Really it depends on the circumstance that they get it on. If they open up their own contest for concepts, for example, they won't have to buy the rights because you're giving your work to them. Similar to the Community Skin Contest, winners get 8000 crystals and 50 codes of their skin to give away.
So for the OP's post, Hi-Rez wouldn't have to buy each concept if the creator say they are giving it for free, like in a contest. However, if they happen to be looking for community concepts and pick one that they like (case with Cuddly Moji), they will purchase the rights and give you some stuff. So if the OP meant something similar to the Skin Contest, they wouldn't have to buy it, but if it's something otherwise and there isn't a "this skin belongs to them when you submit it" part of the deal, they have to buy it.