Resizing an image downwards to not exceed its column width
Causing a browser to resize an image is generally not a good idea, browsers typically do a poor job of resizing.
On the few occasions where it is appropriate to do so you can specify "max-width:100%" for the image, the image will be resized downwards if there isn't enough width available in its column. To get the image to retain its native ratio you should ommit the height attribute for the image in the HTML.
See source code for details such as the hacks used to get this to work in IE.