Thanks for all the replies, I uploaded a cover using the kobo utilities plug-in and now the cover displays full screen in sleep mode as opposed to having the black lines down the side which is fine, however, the image in the library view remains the same
I can't seem to find the cover for the Kepub anywhere, there are plenty in the images folder but they seem to be covers of books that I don't even have. Could these be stored images of recommended books?