'Cause my comic "Story Of A Robot" (http://www.drunkduck.com/Story_of_a_Robot/) has this banner headlining the page and I would really like that banner to change depending on how far a reader is through the comic.
Can I do this?

No, that's not possible with this system, since the HTML page doesn't change, only the image and your author notes. Given that is the case, you could put a banner in your author notes or incorporate the banner into your comic images.

Thanks for your help. Unfortunately, neither of the solutions will be very simple to implement, what with 124 pages to sort. But this has got me a-thinking...

Is there some obscure html code that allows me to state the image source in my author's notes and the image location in the page layout. Or am I just stubbornly clutching at straws now?

No, sorry

You can't do HTML in authors notes, it's only BB code. - [img] and all that guff.