Let me give you a little different code then for that. If you change just one dimension the image will retain it's width to height ratio. I usually change the larger of the two dimensions. In this case I'll change the width.
<img width=200 src="http://www.wdwinfo.com/sites/billk/k4.jpg">
Will get you...
<img width=200 src="http://www.wdwinfo.com/sites/billk/k4.jpg">