For the actual ebooks I tend to keep the covers as they come, unless I have a physical copy of the book and then I will change the ebook cover to match the physical book.
As for the sleep screen of my Kobo I create 'sets' of images along different themes - at the moment it is ancient sites/ ruins i.e. Stonehenge, Petra etc I also have a movie set, Jurassic Park, X files etc and a paranormal set, UFO's etc
