@labanbrown You should definitely take a closer look at the recently updated AdvancedPublisher 4 stack. This sounds like it would be perfect for what you want:
I envisage that you could use AdvancedPublisher to setup a whole series of different banner images, and set one to display each month. You could either use images dragged and dropped into RapidWeaver or warehoused images. Once setup, the system would work all by itself.