Display thumbnail using defineTableAttributes for a custom Element Type
Ok, so I've managed to get an answer to this courtesy of Brandon Kelly.  It's much more simple than I thought.  Firstly, you do not have to specify a relatedTo parameter the defineCriteriaAttributes(). Craft takes care of this automatically.  So in order to retrieve the details of an asset in a table list view you do the following in the getTableAttributeHtml() function in your custom Element Type class:

$relatedAsset = $element-&gt;myAssetsField-&gt;first();


This simple line of code gets a related myAssetsField of your element. I'm not sure why, but this object is placed in an array...  Maybe someone can tell me why this is the case?

I then took the fist element of the array and used a the getThumbUrl() function which is really handy for quickly outputting the url to a thumbnail.  I passed a size of 60 pixels.

case 'myAttribute&gt;':
{   
   if(count($relatedAsset) &gt; 0) {
      return '&lt;img src="'.$relatedAsset[0]-&gt;getThumbUrl(60).'"&gt;';
   }
   return '';
}


The result is a thumbnail in your table list!
Hope this helps others....