I think what you need to change is in admin/images.php:
Here is the current code:
while ($image_row = $site_db->fetch_array($result)) {
echo "<tr class=\"".get_row_bg()."\">";
echo "<td><input type=\"checkbox\" name=\"deleteimages[]\" value=\"".$image_row['image_id']."\" /></td>";
echo "<td><b><a href=\"".$site_sess->url(ROOT_PATH."details.php?".URL_IMAGE_ID."=".$image_row['image_id'])."\" target=_blank>".htmlspecialchars($image_row['image_name'])."</a></b> (".$image_row['image_media_file'];
I tried to add some img src= code to show the thumbnail, but couldn't get the syntax right. Maybe someone else can help?