How to increase page's width

Hi all, i have a wordpress site here http://www.museobianconerocesena.it . If you go to Il Museo -> Decennio 2010-2020 i’d want to post a page that contains the image that is in the page now, but without automatic scaling. I’d want that the Image can continue it’s width until the complete width of the page (it will be perfect for example if i can decide the width of the image).
How i can do to do this?
Have i to modificate template code? How?
Thank you for your help.