I don't think there is an option for displaying the full size, you would have to rescale you images to fit the screen.
The thing is, at least I think so, that the viewer can only scale the images down to 1/2, 1/4, 1/8 etc. of your screen size. That means, if you have a 320x240 pixel screen, a 330x250 pixel image will be very small when you haven't zoomed in on it.
Maybe it could work if you scaled them down to 2, 4 or 8 times the size of you screen. With a bit of luck the images will scale down to fit your screen properly.