Ok, here's a very basic setup i tested in Firebug
function centerImages(){
var imgW=2560;
var mainW= $('#slideshow').width();
var margL=0-(imgW-mainW)/2;
$('.slide-img img').css('marginLeft',margL);
}
centerImages();
You also have issue of window resize to manage, following will recenter, but not sure how you want this handled
$(window).bind('resize',centerImages);