I have written a new Wordpress Plugin which allows an image to be displayed in a widget area at 100% of the width of that widget area. The image should be between 200 and 500 px wide.
I have released the Widget through my web development site, Old Castle Web Solutions. It is free to download and is released under GNU2 license. Some extra work is needed, so that images can be uploaded through the form, and so that images can be edited automatically to the correct width. But, in the meantime, please feel free to try the plugin out.