There are three ways to do this right now:
a) Use the existing ImageField module, you will have trouble then getting the image into the right place in the layout
b) Use URL layers so they can add a banner widget on each page (using either HTML or ImageField again) - this could work quite well if they want the same image across lots of pages, as you can just keep addings URLs into the layer rule
c) Use my Mechanics + Media Garden modules to create a "BannerImage" relationship and use the PaperclipPart to push it into the Header zone
In the next few weeks I'll have an easier way to do this, you'll be able to use my Alchemy module and then layout pushes will be possible directly in Placement.info, so you could go with the ImageField module and just push it to wherever you want on the
page. But that feature isn't quite finished yet, and in any case you need 1.x branch to use Alchemy.