To add a banner image, you must customize the selected theme. You can do this by clicking on the computer and lightning bolt image noted in the picture below.
Once in the new menu, navigate down to banner image option noted by the red arrow below. Here you can upload your own banner image that you have created by using the "add a file" button next to the green arrow. You can use PowerPoint, Photoshop, etc to do this. Start out at a pixel size of 980 x 200. If the theme has a banner image pre-loaded click the red "X" next to it and you can then upload your own. Note that there are also options for adding a boarder to any side of your new banner image.