Hi keyboardji,
As of now, there's no way to achieve this without hacking my plugin's code. This is what you need to do:
# Open wordpress-popular-posts.php using a text editor such as Notepad or Dreamweaver.
# Find (around line 718):
if (!function_exists('get_the_post_thumbnail')) { // if the Featured Image is not active, show default thumbnail
$thumb = "<a href=\"".get_permalink($the_ID)."\" class=\"wppnothumb\" title=\"". $title_attr ."\"><img src=\"". $this->default_thumbnail . "\" alt=\"".$title_attr."\" border=\"0\" class=\"wpp-thumbnail\" width=\"".$tbWidth."\" height=\"".$tbHeight."\" "."/></a>";
} else {
if (has_post_thumbnail( $the_ID )) { // if the post has a thumbnail, get it
$thumb = "<a href=\"".get_permalink($the_ID)."\" title=\"". $title_attr ."\">" . get_the_post_thumbnail($the_ID, array($tbWidth, $tbHeight), array('class' => 'wpp-thumbnail', 'alt' => $title_attr, 'title' => $title_attr) ) . "</a>";
} else { // try to generate a post thumbnail from first image attached to post. If it fails, use default thumbnail
$thumb = "<a href=\"".get_permalink($the_ID)."\" title=\"". $title_attr ."\">" . $this->generate_post_thumbnail($the_ID, array($tbWidth, $tbHeight), array('class' => 'wpp-thumbnail', 'alt' => $title_attr, 'title' => $title_attr) ) ."</a>";
}
}
# ... and replace with:
if (!function_exists('get_the_post_thumbnail')) { // if the Featured Image is not active, show default thumbnail
$thumb = "";
} else {
if (has_post_thumbnail( $the_ID )) { // if the post has a thumbnail, get it
$thumb = "<a href=\"".get_permalink($the_ID)."\" title=\"". $title_attr ."\">" . get_the_post_thumbnail($the_ID, array($tbWidth, $tbHeight), array('class' => 'wpp-thumbnail', 'alt' => $title_attr, 'title' => $title_attr) ) . "</a>";
} else { // try to generate a post thumbnail from first image attached to post. If it fails, use default thumbnail
$thumb = "";
}
}
# Save changes.