The site builder offers a selection of styles that allow you to change the banner image. If the style you select supports a custom banner image, a blue box with the title "Edit Banner Image" will display on the title bar when you mouseover the banner image.
To change the banner image, you can click directly on the banner area or go to Style > Style Settings > Style Designer. The Style Designer will open up on the left-hand side.
Under "Colors", click on "Banner Image".
Each banner has recommended dimensions which can be used to achieve the best result. Although you can edit the banner image once it has been uploaded, it's best to upload an image that matches the recommended banner dimensions.
To find the banner dimensions, look to the top of your banner area that appears when you mouseover within the banner area. On this particular style, you can see that the recommended banner size is 1280 x 540px.
Banner images can be created on your computer using a simple image editor or you can use more advanced graphic software such as Photoshop or Gimp.
There are quite a few banner making programs on the internet if you would like to create your own. Two popular options are Banner Fans and Banner Maker Pro.