I'd really like to have automatically rotating image headers, to give readers a different experience each time they check in. You can see an example on Chris Pearson's award-winning tech site at http://www.pearsonified.com
That doesn't seem to be possible on WordPress.com, at least not yet. So as a halfway house for now, on one of my site I like to update my image header each time I post. It serves as a kind of code so that regular readers can see there's new material and a new theme.
The present header image reflects a series of posts I'm writing to draw attention to poverty in Africa, and what we can do about it:
My other site is much more focused on a single theme. There I prefer to use a static header image, at least for now -
- conveying the message of hope at the end of the journey.