Yeah like Tiny said. remember to click 'choose' under the image instead of clicking the actual image. I've done that a few times. If the change doesn't happen immediately, hold down Ctrl and press F5 and it'll appear.

when you finished your fanfic, you´ll see a button saying "Edit the cover picture" click it and then upload your cover picture from your computer there. You´ll see it´s the same like sending a fanart/image.

Hope it helps

Jordan.

go to change my gallery option change the cover image, i believe,