When I contemplated changing my blog layout width it also meant changing the banner. I have seen blog design guidelines that recommend including a mug-shot to "personalise the brand". To test this I did a few "sketch concepts after configuring the new layout on my test blog.
Although I use CorelDRAW to create the real banners I did these mock-ups with Snagit. At the time I was using a Tablet PC and editing with a pen worked really well. I started with a screen-shot but the Snagit Editor proved to be a handy design tool. As you can see, above, i didn't use this format but Snagit was a quick way to try the concept.