The most commonly used JavaScript effect on the Web is the image rollover. For the one person with access to the Web who’s never seen one before, an image rollover happens when you move your ...

after the image has completed loading. The following example illustrates this by displaying a “please wait” screen while the image loads, and then ...

The image probably isn't loaded by that time. I mean, it's probably a couple milliseconds, if that, between when you assign that src and when you ask for the width and height.