If your entire book is the same format of a single image per page, then the CSS becomes really easy:
div {margin:0 auto; text-align:center}
img {
height:800px; width:800px; max-height:99%; max-width:99%}
And your HTML becomes:
<body>
<div><img alt="" src="../Images/xxxx.jpg" /></div>
</body>
You would only need specific classes for the few front/back pages (cover,title,acknowledgement,dedication,copyright, etc.) if you use a div or img on them.
You don't need to put the image file name in the alt tag. That is supposed to be used as a description of the image ("Goat seen standing on top of cloud, two pilots looking out windshield wondering what a goat is doing in the clouds") if the device can't render the image for whatever reason (missing file, image display turned off, etc) In your case you will make sure all the pictures are included in the ePub, and no one will turn off image rendering in a picture book!
I also use 99%, instead if 100%, to avoid those rare instances when a picture might bleed over onto a separate page.
I hope that helps!
Cheers,
[edit] On second thought, there is no need to define the width or height, it will automatically display at the full size unless limited by the max tags.