In Catch Box Pro theme you can just add the following css in your “Theme Options” panel to add background. Change the image url to your image url
#branding {
background-image: url("http://i2.wp.com/catchthemes.com/demo/catchbox-pro/files/2012/12/edit-IMG_8596.jpg?resize=644%2C320");
}
There is another way as well, Just upload the large image in “Appearance => Header” and then remove the margin in “Appearance => Theme Options => Header Options”.