WordPress Quick Tip #4 - Automatically Set the First Post Image as a Featured Image

Use the simple script listed below. We recommend placing it in the theme's functions.php file:

function auto_featured_image() {
    global $post;

    if (!has_post_thumbnail($post->ID)) {
        $attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );
        
	  if ($attached_image) {
              foreach ($attached_image as $attachment_id => $attachment) {
                   set_post_thumbnail($post->ID, $attachment_id);
              }
         }
    }
}
// Use it temporary to generate all featured images
add_action('the_post', 'auto_featured_image');
// Used for new posts
add_action('save_post', 'auto_featured_image');
add_action('draft_to_publish', 'auto_featured_image');
add_action('new_to_publish', 'auto_featured_image');
add_action('pending_to_publish', 'auto_featured_image');
add_action('future_to_publish', 'auto_featured_image');

This script is very simple - it checks if the specific post has a featured image set and, if the featured image isn't set, the script checks for the first attached image and sets it as a featured image. This operation is executed for every save or post operation.

As this instruction is executed on every post displayed, introducing a slight performance hit to your website, we recommend removing the following lines once all the featured images have been generated:

// Use it temporary to generate all featured images
add_action('the_post', 'auto_featured_image');

Here are two important details to take away from this:

1. The featured image must be removed from the media manager.
2. The attached image must first be used in the specific article; if the image was used and attached to a different post, it will not be set as a featured image for the other post.

It is for this reason I recommend only using this method to migrate your older posts in the old format, and once done setting the featured image feature manually.