Wordpress - Get the Image from Post Thumbnail or the First Image of a post or else use a default Image.

wordpress-post-thumbnail-or-first-image-or-default.php

/*
 * Display Image from the_post_thumbnail or the first image of a post else display a default Image
 * Chose the size from "thumbnail", "medium", "large", "full" or your own defined size using filters.
 * USAGE: <?php echo my_image_display(); ?>
 */

function my_image_display($size = 'full') {
	if (has_post_thumbnail()) {
		$image_id = get_post_thumbnail_id();
		$image_url = wp_get_attachment_image_src($image_id, $size);
		$image_url = $image_url[0];
	} else {
		global $post, $posts;
		$image_url = '';
		ob_start();
		ob_end_clean();
		$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
		$image_url = $matches [1] [0];
		
		//Defines a default image
		if(empty($image_url)){
			$image_url = get_bloginfo('template_url') . "/img/default.jpg";
		}
	}
	return $image_url;
}

before the last if statement, if first_img is empty then you could just do preg_match_all('/[img].+'"['"].*[/\img]/', $post->post_content, $matches2);)
$first_img = $matches2[1][0]