Assuming the brunt of your artwork was on the left hand side of the image you could just make the background overly large so that any larger resolutions see a wider range of the right hand side of the image. However you'd need to use CSS in order to make the image a part of a background to an element such as a div.
It could be done, using Javascript or PHP, however it's a trifle complicated. More than I feel like typing at 20 to 1 in the morning.
Or you might get away with a table background, and rotate your images using the script with a few small adjustments.
Also, you may wish to put your code within the container divs so that it doesn't overlap with the edges of the background as it does now. Just after this section of code:
Code:
<div align="center">
<div class="page" style="width:950px; text-align:left">
<div style="padding:0px 25px 0px 25px">