When you do fan art based comps in the future, place a waiver in the rules, that states that whilst the original artist retains the IP for the artwork, submisssion of artwork to MS grants them right for images to be used in non commercial forms, but not limited to promotions, advertisment etc.. Its a standard waiver for this kind of competition, its basically an agreement that the competition sponsor can use the images or derivities in any promo work, but are not allowed to sell the original or any derivitive work.