functions.php

<?php

/**
 * Get post image.
 */
function wds_get_post_image( $size = 'thumbnail' ) {

	// If featured image is present, use that
	if ( has_post_thumbnail() ) {
		return get_the_post_thumbnail( $size );
	}

	// Check for any attached image
	$media = get_attached_media( 'image', get_the_ID() );

	// Set up default image path
	$media_url = get_stylesheet_directory_uri() . '/imgs/default-post-thumbnail.png';

	// If an image is present, then use it as a thumbnail
	if ( is_array( $media ) && 0 < count( $media ) ) {
		$media     = current( $media );
		$media_url = ( 'thumbnail' === $size ) ? wp_get_attachment_thumb_url( $media->ID ) : wp_get_attachment_url( $media->ID );
	}

	return '<a href="' . esc_url( get_the_permalink() ) . '"><img src="' . esc_url( $media_url ) . '" alt="" /></a>';
}

Thanks. I've updated mine with (tested) code. https://gist.github.com/gregrickaby/dbc224eabbf3707d8c5d