Image from Post (Feat'd, Attach'd, 1st, YouTube, Default)

<?php

/*
 * This function will get the first image in this order:
 *
 *    1) Featured image
 *    2) First image attached to post
 *    3) First image URL in the content of the post
 *    4) YouTube screenshot
 *    5) Default images for different categories [SET MANUALLY]
 *    6) Backup-default image if all else fails [SET MANUALLY]
 *
 * A big THANKS goes out to Michelle @ WordPress.StackExchange.com
 * for her answer (http://bit.ly/O3UaLm) that made this possible!!!
 *
 */

//
// Activate post-thumbnails by adding this to theme's function.php file
//
add_theme_support('post-thumbnails');


//
// Add the following to the theme's function.php file
//
function vp_get_thumb_url($text, $size){
    global $post;
    $imageurl="";

// 1) FEATURED IMAGE
    // Check to see which image is set as "Featured Image"
    $featuredimg = get_post_thumbnail_id($post->ID);
    // Get source for featured image
    $img_src = wp_get_attachment_image_src($featuredimg, $size);
    // Set $imageurl to Featured Image
    $imageurl=$img_src[0];

// 2) 1ST ATTACHED IMAGE IMAGE
    // If there is no "Featured Image" set, move on and get the first image attached to the post
    if (!$imageurl) {
        // Extract the thumbnail from the first attached imaged
        $allimages =&get_children('post_type=attachment&post_mime_type=image&post_parent=' . $post->ID );

        foreach ($allimages as $img){
            $img_src = wp_get_attachment_image_src($img->ID, $size);
            break;
        }
        // Set $imageurl to first attached image
        $imageurl=$img_src[0];
    }

// 3) 1ST IMAGE URL IN POST
    // If there is no image attached to the post, look for anything that looks like an image and get that
    if (!$imageurl) {
        preg_match('/<\s*img [^\>]*src\s*=\s*[\""\']?([^\""\'>]*)/i' ,  $text, $matches);
        $imageurl=$matches[1];
    }

// 4) YOUTUBE SCREENSHOT
    // If there's no image attached or inserted in the post, look for a YouTube video
    if (!$imageurl){
        // look for traditional youtube.com url from address bar
        preg_match("/([a-zA-Z0-9\-\_]+\.|)youtube\.com\/watch(\?v\=|\/v\/)([a-zA-Z0-9\-\_]{11})([^<\s]*)/", $text, $matches2);
        $youtubeurl = $matches2[0];
        $videokey = $matches2[3];
    if (!$youtubeurl) {
        // look for youtu.be 'embed' url
        preg_match("/([a-zA-Z0-9\-\_]+\.|)youtu\.be\/([a-zA-Z0-9\-\_]{11})([^<\s]*)/", $text, $matches2);
        $youtubeurl = $matches2[0];
        $videokey = $matches2[2];
    }
    if ($youtubeurl)
        // Get the thumbnail YouTube automatically generates
        // '0' is the biggest version, use 1 2 or 3 for smaller versions
        $imageurl = "http://i.ytimg.com/vi/{$videokey}/0.jpg";
    }
// 4) YOUTUBE SCREENSHOT
    // If there's no YouTube video in the post, look for the default based on the category
    if (!$imageurl) {
        // Set default Image for different categories
        // [SET DIRECTORY MANUALLY!!]
        $dir = get_template_directory_uri() . '/images/'; // [SET MANUALLY!!!]
        // [DID YOU SET YOUR DIRECTORY?!]

        $get_cat = get_the_category();
        $cat = $get_cat[0]->
        slug;
        // [SET IMG EXT MANUALLY!!]
        $imageurl = $dir . $cat . '.jpg'; // [SET MANUALLY!!!]
        // [DID YOU SET YOUR IMG EXT?!]

        // Use this array if you have a few main categories that you want images for
        $array = array( 'cat_1', 'cat_2', 'cat_3',);
        if (!in_array($cat, $array))
            // [SET BACKUP IMAGE MANUALLY!!!]
            $imageurl = $dir . 'district.jpg'; // [SET MANUALLY!!!]
            // [DID YOU SET YOUR BACKUP IMAGE?!]
    }

    // Spit out the image path
    return $imageurl;
}


//
// Add inside loop but before displaying image
//
if (function_exists('vp_get_thumb_url')) {
    // Set the desired image size.
    // Swap out 'thumbnail' for 'medium', 'large', or '[your-custom-size]'
    $thumb=vp_get_thumb_url($post->post_content, 'large-feature');
}


//
// Call the image inside the loop
//
?>
<a href="<?php get_permalink(); ?>" title="<?php the_title_attribute(); ?>" class="featured-image">
    <img src="<?php
        // Gets the image URL
        if ($thumb!='') echo $thumb;
    ?>" class="attachment-thumbnail wp-post-image" />
</a>

<?php
/*
 * A big THANKS goes out to Michelle @ WordPress.StackExchange.com
 * for her answer (http://bit.ly/O3UaLm) that made this possible!!!
 *
 */
?>