Image Module: Image Styles: Need to set image width to 100%

I have spent over two hours tonight searching the Drupal site, and Groups, especially this one, to no avail. My last resort is to create this discussion.

I have a Content Type with an image field defined. I am able to apply an image style to this if I wish, using Image Styles. However, Image Styles only allows specification of width and height in pixels. How can I set width=100%?

All help gratefully received.

TIA,

bbater