OK, I've answered my own question. First I made a new test book using the same original template (contemporary). I made two chapters and put a different image on each of the chapter's first page. In the TOC, those images also show up. So the image on the first page of the chapter is linked to the image in the TOC.
Then I used that info to troubleshoot my own problem because those images were only showing up correctly in Chapter 1. Each chapter in my book has a slightly different color tint to it's layout. I had duplicated the Chapter 1 layout and modified it for a chapter 2 layout etc... I think that somehow I had deleted the placeholder media in the first duplicate, and though I had images, they were statically placed images. Everything seemed to work except when the TOC tried to link up with the image of each chapter's first page.
To solve the problem, I re-duplicated the first chapter's layout, modified it, making sure that I got the new images in the placeholder, re-applied it to Chapter 2 and voila, the TOC showed the image correctly.
The moral is that if you muck about in the layout templates, make sure that you don't delete something that is linked downstream to other pages. We will obviously learn more about this in time.