The reason why it does show up properly in certain screen resolution is because your image dimension is 1550x417, and you using CSS background position property to set it to 1300px 400px. That's not a very responsive way to setting background position, and it bound to vary from screen to screen.
There is an easier way to do this. Since the banner is mostly solid color why not slice only the graphic part of the image, and use background color? You can then position the image to the center of the element.
Slice: http://i.imgur.com/2s35JYs.png
Example: background: url('../images/banner2.jpg') center center no-repeat #ECE8DF;