Using background images will allow you to easily centre the images and maintain aspect ratio:
html, body {
height: 100%;
}
#top, #bot {
height: 50%;
}
#top {
background: url(/your/image/path.jpg) center;
background-size: cover;
}
#bot {
background: url(/your/image/path.jpg) center;
background-size: cover;
}