Is it possible to include a thumbnail in an element type index view?
(Hopefully it is okay for me to answer my own question)

In the ElementType file, the getSources method needs to have the hasThumbs key set, as follows:

public function getSources($context = null)
{
    $sources = array(
        '*' =&gt; array(
            'label'    =&gt; Craft::t('All assets'),
            'hasThumbs' =&gt; true,
        )
    );
}


In your Model file you have to have a getThumbUrl method defined, as follows:

public function getThumbUrl()
{
    return $this-&gt;thumbnailUrl;
}


This results in a small thumbnail next to the title of your element, as seen in the screenshot:



The Craft 2 Class Reference helped put this together for me.