Funny you should say that. The immediately adjoining thread in List Unread currently ends with
Don't for one second believe that pages which aren't indexed can't affect your rankings. This was an eye opening exercise.
a different header image based on topic, the heading would be redundant
You can put the image inside <h1> tags and then the alt will display with your header format if there's a problem. (The SEO sites may complain about this, but g### itself doesn't, so what do they know.)
This is also a pretty dramatic way of reminding you that you forgot to upload the image. Ahem.