Image width problem

Hi I got a design that is 1024 px wide, and I want a image (on a page) to be the same width, but it dosen't happen.

I've made the Image 1024 px wide in PhotoShop, and when I insert it on a page I click the full size button, but when I prewiev the site the image is about 990 px, and not as wide as the text.

Hope it makes sense :-)

Mars, Copenhagen