From the 'Website' menu, you have access to many different options to change the look of your group. The 'Theme' page includes a way to upload an image to be used as a banner above your tabs bar.
The recommended header image width is 955 pixels. If you want to have your image centered, try creating an image that is 955 pixels wide and placing your smaller image in the centre of that before uploading.
Allowed image formats are JPG, GIF and PNG. The maximum allowed size of your image is 128MB. If your upload doesn't work, try uploading a smaller one.