Yep, try 'display_helper' => 'ui/thumbnail' in your column's definition array.

And create a new helper called "ui/thumbnail", put into it the helper code from the "image/thumbnail" in this package:

http://podscms.org/packages/image-sizes/

Then modify the new helper to fit your needs, or maybe it doesn't need any modification.

Cool, did not know that I could use helpers is this way. You opened my eyes. I solved the problem by creating this little helper:

<?php
if($value){
  $img = wp_get_attachment_image_src($value[0]['ID'], 'thumbnail');
  echo "<img src='{$img[0]}' width='80' />";
}
?>

You saved my life, thanks!