How to allow the banner to be editable via front end?
You have two ways without coding:


Use Nodeblock to create nodes for banners. Give users right to edit nodes and place them where you need them as a blocks.
Use Views to create a block displaying (random?) node of content type banner. Give your users right to create, delete and edit them.


You could also use Advertisement module, but for a simple case like this it's an overkill, I think.