Code Works On Reload But Not On Initial Load

Basically, what I'm trying to do is set the left margin of an image based on the width of the image, because I want the image to appear centered relative to the link table above. I've been using this code:
function insertImageMargin(){
var marg = getImageMargin();
document.getElementById("comic").style.marginLeft = marg + "px";
} function getImageMargin(){
var com = document.getElementById("comic");
return 440 - com.width / 2;
}
What happens is, the first time com.width is 0, but if I hit reload, it is the width of the image, and the code works correctly. How do I get this to work the first time.