I know I'm really late to reply to the first message but:
Go into your works, click on the current cover of the story whose cover you wish to change. (Basically click on the picture to the side of the story you wanna change the cover of)
Then Hit 'upload' if you have a saved document that you want as the cover or hit 'create' if you just want a draft cover or something. You can ask people to make covers, save them the upload them onto a story, that's what I do.