Not that I drew an actual cover or anything yet, but HALP I can't figure out how to change the cover again and it might be stuck as perma ika musume D; (in hindsight I have no clue why I didn't just leave it blank or use page 1)
@Orio
Because of the way Mangadex is set up, you would have to be a "Group Leader" to edit any details relating to the manga title entry.
But another easier option is to simply Report your entry and ask the moderators to update your cover with whatever image link you send to them.