I checked http://wordonthe-street.com/ and I see you've added some custom CSS to replace the header image:
background: url('http://wordonthestreetdotcom.files.wordpress.com/2012/11/cropped-cropped-wordonthestreesceene2-01.jpg') 0 0 no-repeat;
The space for a header is 920px while the image you've references is 885px:
So there's some extra white space to the right of the header. To get rid of that, you can create a new header image that is 920 pixels wide, upload it to your media library, copy the image URL, and use that in the url() field in your custom CSS.
You can also make the min-height value match the height of the new image you upload to prevent extra white space between the header and the main menu.