From your administration page select the "Design"->"Theme"->"Banner Images".To change an image, simply click on the image to open the image manager. Click on"Choose File" to upload or select from album.You can upload as many images as you want. When you have finished uploading your photos, then simply double click on the photo you want to use as your banner. Once you have uploaded an image into the image manager, it will stay there and can be used for any future changes.