making a book cover is somewhat a matter of graphic design. you have to have a knack for combinding pictures and different graphics and whatnot. :D
but when i do covers, i go on picnik cos i have a little membership thingy. but i just combine pictures and effects and a title. there's a whole bunch of people here on wattpad who are GREAT at making them too!
but yeah, you just have to have a knack for it