[glsp]

I had the opposite problem, where I needed the popup window to match the height of the image better. Here is my code at the end of skin1/main/popup_image_js.js:

function imgOnLoad(obj) {
	if (images.length == 0 && obj)
		imgObj = obj;
	
	winH = imgObj.height+100<screen.height?imgObj.height+100:screen.height;
	window.resizeTo(imgObj.width, winH);

	expandWindowX(false, 10, screen.width);
	//expandWindowY(false, 10, screen.height);

	if (images.length == 0)
		return true;

	imgObj.alt = images[current_id][1];

	return true;
}

The original has
window.resizeTo(imgObj.width, imgObj.height);
and you could just change those to whatever dimensions you want.