Cómo capturar primera imagen de post en WordPress

# Displays post image attachment (sizes: thumbnail, medium, full)
function dp_attachment_image($postid=0, $size='thumbnail', $attributes='')
{
 if ($postid<1)
  $postid = get_the_ID();
 if ($images = get_children(array(
		'post_parent' => $postid,
		'post_type' => 'attachment',
		'numberposts' => 1,
		'post_mime_type' => 'image',)))

 foreach($images as $image)
 {
  $attachment=wp_get_attachment_image_src($image->ID, $size);
 }
 else
  $attachment[0] = 'http://www.mi-url.com/sin-imagen.jpg';
  ?>
<img src="<?php echo $attachment[0]; ?>" alt="<?php echo get_the_title($postid)?>" title="<?php echo get_the_title($postid)?>" <?php echo $attributes; ?> >
<?php
}