Not all of our templates have customizable banners. If you want to be able to change the banner image, you will need to select a template that supports this option.
If a template has a customizable banner, a blue box with the title Edit Banner Image will display when you mouseover the banner image. Then, the dimensions of the banner area will be displayed at the bottom of the dialog box when you click on the banner area. This will allow you to see what sizes are needed to properly create your banner.
If you do not see Edit Banner Image option when you mouseover the banner area, this means that the template you have selected does not support a custom banner image.