Sorry, the things you are having problems with are inherent in the kepub format. The header and footers are part of the design of the kepub renderer on the device. Hence the space for the page is smaller and the image is smaller.
But, there is a unsupported fullscreen mode. Add the following lines to the "Kobo eReader.conf" file:
Code:
[FeatureSettings]
FullScreenReading=true
There are problems with this mode, but I don't know how they will affect an image only kepub.
Is there any reason not to use CBR or CBZ? The Kobo devices support both formats including zoom on and rotate on the images.