Finding an Image's width/height dimensions using JavaScript

For a complex Drupal node form I've been working on for flocknote, I have a relatively complicated image switching functionality that lets people change an imagefield on the node (either when creating a new one or editing an existing node), and once the imagefield is changed, some custom jQuery code will grab that image and display it in the form, for a very WYSWIYG-like experience (the node looks almost exactly the same when editing/adding as it does once the user saves the node).

One problem is that images can be arbitrarily high (though they're resized to 600px wide), and I can't easily get the height of the image through any traditional means. If I were grabbing an already-saved imagefield image, I could throw the image height into the JS settings for the page. However, getting a dynamically-added image's height/width values is surprisingly tricky using JavaScript, at least if you take a look around the web and try using many people's suggestions (which work great if the image was already loaded with the page's content, but not if the image is dynamically added, or if the image hasn't yet loaded on the page.