[SOLVED] How does Montezuma select the image for each blog post

First, I apologize if this question has been asked and answered somewhere. I search and didn't see anything. Second, absolutely love the Montezuma theme. I'm new at using it and have a new blog, so haven't made any customizations, but love it right out of the box.

That said, sometimes my blog posts will have multiple images. And as you know, on the index page, above each blog post a B&W version of the image will appear. My question is, how does Montezuma select the image that appears there? My first blog post was fine, it only had one image. My second blog post has one image, but also am using Zemanta's related articles, which have images. For some reason, one of the Zemanta images was selected. I actually don't mind the Zemanta image selected, it is just very small for that bar.

You can see what I mean at blog.gregoryjwright.com

So is there a special way to set which image appears in this bar? Thanks in advance for your help!

GW