IE8 non-compatibility mode, image with max-width and height:auto

I have an image with this markup

<img src="wedding_00.jpg" width="900" height="600" />

And I am using CSS to downsize it to 600px width, like so:

img {
    max-width:600px;
    height:auto;
}

Can anyone explain why this method works in Compatibility mode, but not in standard mode? Is there a way I can modify my CSS so that it will work in standard mode?

I realize that if I strip out the

width="900" height="600"

that it solves the problem, but that is not an option I have.