Okay! :D You know how you can go to your 'my mibba?' well, click that and then look over to the green and blue stuff on the right side, go down to orange that says 'stories' and click the blue link that says 'stories.' and once you've clicked it, you'll be taken to your list of stories. this will show you all your stories and comments and chapters, ect. Then, right next to the stories should be a big ? mark picture box right next to it. You just click that big ? picture, it's right under the section called cover ^_^ once you get there, you can browse through your photos and once you've picked it, click submit and now you have a cover! :)
I hope I was able to help and if you need anything else, just let me know ^_^