To modify the height or width in this way, you'll have to use a different code for displaying the image. Plus, to make sure that the ratio of height to width remains constant, it is easier to just modify one of the dimensions and the other will adjust accordingly. Usually for a signature image, take the larger of the dimensions and set it to 200. So, for your picture, the code would be....
<img width="200" src="http://www.thewarnerfamily.homestead.com/files/AKL-K_P-Njoki_Thabo.jpg">
and this will give you...
<img width="200" src="http://www.thewarnerfamily.homestead.com/files/AKL-K_P-Njoki_Thabo.jpg">