i'm guessing he's on a 15" macbook pro which is (typically) 1440px wide. i'm on the same, and the image isn't quite wide enough to fill the browser. no big deal.
looks great though, and am quite tempted to give it a go. just to need to find a good pic to use.
if you upload a picture that is 1440px+ wide it will work. the limitation is the size of the picture.