see
this thread.
There are two ways of setting an image to display using the full page, using either the CSS or using SVG. SVG is a bit safer as it will still work (when the proper values are inserted in the code) no matter what the aspect ratio of the image is. The CSS method is more portable, in case you decide you want to convert your ePub to .mobi, but it assumes that your image has a portrait aspect ratio (which is generally true for covers, but not always).