This must be simple I am sure.....How do I get a Category Image to be the width of the right hand side of the page (ie to the right of the category list)?

I have tried various settings in Admin/Configure/Images/category icon image width and height (currently set to 800x 280). I have even tried resizing the image prior to uploading, but cannot get it to display bigger than eg Little Darlings image.
http://holkat.co.uk/index.php?main_page=index&cPath=123

I need it to stretch right across even if it goes further down the page.

thanks