Preloaded Image Won't Display

I'm working on a web map, in which you click on a building to open a pop-up with a photo of that building in the window. Rather than use a generic pop-up, I've styled it to look similar to the parent page. Since my map contains over 200 buildings, initially this meant creating over 200 separate html pages with the image source and dimensions for each building photo. That's clearly very inefficient and not a process I want to duplicate! Therefore, I needed some dynamic code to generate the content in the pop-up.

I successfully used a cookie to pass a building ID variable to the pop-up and use that to grab the associated photo from a directory and that worked fine. However, I ran into problems because the photos are not identical in size, and I also needed to get the image width and height. I tried preloading the image using a standard image caching script and used that to obtain the width and height of the image. Then, I used document.write to place those variable in the <img> tag and place the photo on the page.

The problem is that the cached image will not display on the page when it loads, only if I hit refresh. Can anyone tell me how to get around this? Here's the code I'm using. I call this function inside a <p> tag in the body of the page:

if (document.images) {
bldg = new Image();
bldg.src = "../../Columbia/bl/bl_" + BldgId + ".jpg";

theWidth = bldg.width;
theHeight = bldg.height; }

function showImage() {
var photoSrc = "<img src='../../Columbia/bl/bl_" + BldgId + ".jpg'";
var insertPhoto = photoSrc + " width='" + theWidth + "px' height='" + theHeight + "px' alt='" + BldgId + " Photo not available' />";

document.write(insertPhoto);}

Is there another way to get the image's dimensions BEFORE I place it on the page?