You can post them in General Discussion. Skins concepts/ideas get posted there from time to time, bonus points if you post them with cool pictures.
Also you can check out leaguecraft.com They've got a thread where you can post skin suggestions, and if some artists there likes it enough they will make a skin, and you can use it as a custom skin (you can see it but no one else will be able to).
Don't get your hopes up about getting Riot to make one though. Not to say it doesn't ever happen (Gentleman Cho'Gath) but its not too likely.