I am sorry that I didn't express properly what I want.
http://i60.tinypic.com/14170oz.jpg
This is how koreader looks while I am reading a book. As you can see, it doesn't have/has very small white borders on the side
http://i61.tinypic.com/24pdy51.jpg
This is how my reader looks like when it is turned off. The picture is selfmade and is exactly 1040x1387 pixel big, however it has very big white borders on all sides, and I can't make it any bigger, as this would just make the frame appear.
I guess the only possible solution is the one you described tshering.