Preload Images Not Working

I'm preloading images with the standard code:

default1 = new Image(); default1.src = "images/image1.gif";
changed1 = new Image(); changed1.src = "images/image1-on.gif";
default2 = new Image(); default2.src = "images/image2.gif";
changed2 = new Image(); changed2.src = "images/image2-on.gif";

And this is my rollover code:

var iName="";
function Ichange(p) {
var pSrc=eval(p+ ".src");
document[iName].src = pSrc;
}

But for some reason on the first mouseover it doesn't seem like the
image has been preloaded. There is a delay, like it's looking for the
rollover image, which should have already been preloaded. Anyone have
any ideas why?

Here is the inline code for a mouseover: