we setup postcard type templates for clients for focused campaigns
basically one image that they can then change
eg a sale, a new brand, one off announcements
but of course they can only link this image to one URL
Was thinking it would be very cool if the client could upload one image and then link different parts of it to different pages
a great use of this would be for ecommerce stores
where the client could lay out a selection of products in the one graphic and then link each product to the respective product page.
Obviously you can create an image map quite easily, however it wouldn't be too suitable for template purposes. Something to consider too is the impartial email client support for image maps, such as Gmail.
Here's a link that may provide more insight. It's not something I'd recommend doing either and comes across as lazy coding:
As Jordan mentioned, it is really not your best option for an email newsletter - much better to have actual HTML text as well as images.
See http://www.campaignmonitor.com/design-guidelines for our general advice on designing for email.