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;
}

Could you please show an example of how to apply new image sizes filters ?
Also, after testing it doesn´t work if the post have more than one image :(

Need some help ...
My blog posts/articles, have [img]image-URL[/img] shorcode created in functions.php theme file.

In my blog posts, I use images with this [img] shortcode, something like this:
[img]http://mysite/images/myimage.jpg[/img]

How to make this "first-image" function to get first image from [img]xxx[/img] tag, if I don't have usual <img src="" /> in my blog post?

This is my code:

function catch_first_image() {
  global $post, $posts;
  $first_img = '';
  ob_start();
  ob_end_clean();
  $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
  $first_img = $matches [1] [0];

  if(empty($first_img)){ //Defines a default image
    $first_img = "/images/default.jpg";
  }
  return $first_img;
}

Thank you.

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]