This tutorial here is what you'll be needing: http://doc.silverstripe.com/doku.php?id=tutorial:2-extending-a-basic-site
It explains how to create image uploaders for pages within the CMS. More specifically, it's the 'Adding a staff section' portion of the tutorial you'll be most interested in.
If you're looking to get a random image instead of a fixed one per page, then you could add the images to a directory inside assets/banners or similar (in the same directory level as sapphire and cms), then create a function as you would normally in any PHP implementation to fetch this directory's contents using scandir or similar, then shuffle the array around.
The banners directory inside assets can be maintained by the CMS using the Files & Images section. The SilverStripe admin can upload the image files directly into the banner directory itself which gets added to the shuffle.
If you haven't already done so I'd recommend running through the tutorials to see how SilverStripe works. :-)
Cheers,
Sean