This is because the width and height attributes of an img are (pixel) values, so "px" in the height attribute is also invalid.
The previous post confirms that you can set these values in CSS.
An alternative approach is to make the image the background to another element. However, stretching a background-image is only available from CSS3:
background-size: 100%; /* CSS3, or */
background: url(images/bg.jpg) no-repeat center center fixed;
} /* IE 9+ */