Javascript Image Objects Not Loaded

I have this...

<div id="mainframe">
<img src="image.jpg" id="image">
</div>

Then when I try to in a function say...

function checkImage()
{
var image = document.getElementById("image");
var height = image.height;
}

Many times the image height will not be defined.

I recently changed the past so it uses AJAX to change the <img src...
stuff to return new data for that inner HTML block..

Still the image will not have properties.
Now, this is a problem in IE/FF, but not in opera.

Any ideas on how to make sure the image is loaded so its height/width
properties are available... either by swapping the image.src or by
using ajax to swap out the div innerHTML??