Let us look at the rest of the readyState==4 conditional branch as there is a bit going on there.

Line 1: getImageSize = document.getElementsByTagName("img")[0] .getAttribute("width");

Line 2: gallery.style.left = (pageWidth - getImageSize) / 2+"px";

Line 3: preload.style.left = (pageWidth - getImageSize) / 2+"px";

Line 4: document.getElementById('captionLYR').style. left = (pageWidth -   getImageSize) / 2 +"px";

Line 5: document.getElementById('accessLYR').style. visibility = "hidden";