Here's one way to handle that (among many... umbraco is very flexible!)
Create a new document type called "promotion" (or some such) and give it at least three properties: an upload field (for the image to use with this promotion), and two date pickers (for the date to start the promotion and the date to end the promotion).
Create a content node called "Promotions" (or some such). This node doesn't need a template because it will never be shown directly. This will be based on the "promotion" document type.
Put your default promotional image in the "Promotions" content node. The start and end dates don't need to be set. This image will be used unless another promotion is current.
Beneath the Promotions node, create additional nodes of doctype promotion. Each of these will have an image and start/end dates according to the promotion.
Users will be able to create/modify promotions easily this way and you won't have to be involved later :o)
Next, create an XSLT macro that takes a ContentTree parameter. When you insert the macro into your template, select the Promotions content node, which is where all the promotions data is stored.
Inside the XSLT file, cycle through all the nodes in the promotions area, sorted by end date. If the end date is equal to "today" or higher, check the start date to see if it has begun yet. If so, show the associated image from that node. If no promotions are active, show the image from the top node.
Umbraco has some date handling functions built-in, and the EXSLT methods have some more.
cheers,
doug.
MVP 2007-2009 - Official Umbraco Trainer for North America -
Percipient Studios