If you have designed a theme to include a specific image within the banner, you should be able to change the img src which the theme pulls from. However, this change will need to be done in the theme, there will not be any options within Jive to make that that change.
However, if you are looking to add an image to the entirety of the banner, you can do so from within Jive. This document goes over the steps to do that Adding a New Image to the Place Banner Presets
Hope this helps.