640 Image Width Reduction

The images I create for my post/page headers are designed at 700px wide. The images are being reduced to fit 640px wide for some reason and that is throwing off the balance between the text space and the images. I would like to fill my content width with a header image for some posts and pages, as you can see I’ve attempted here: http://www.bobbyroach.com/2014/05/transformational-imaging-campaign/

Any help will be greatly appreciated!!