Indexhibit Support

I bet your Media Source is set to 'source'.
Which means that the first image (or it's cover image) of each exhibit under that section, will be placed as a link in the exhibit area.
To use the section page as you would normally do with an exhibit, change Media Source to 'exhibit'.

Ah, that sounds like a bug - I'll add it to the list.

Another way to add background images would be via CSS. There are many CSS classes in the body that you can utilize - in your case you would want to use the section identifiers.