Conditional Statement – check if post has an attachment image

I’m using this code below, it’s quite a bit, but it basically loops through post thumbnail, attached image, image in content and then to an image per category (which is a fixed set on my site, hence the naming scheme of the images in get_post_category_image. The main usage is through the last function, simply do echo get_post_image():

function get_post_content_thumbnail($num, $width="", $height="") {
    $content = get_the_content();
    $count = substr_count($content, '<img');
    $start = 0;
    for($i=1;$i<=$count;$i++) {
        $imgBeg = strpos($content, '<img', $start);
        $post = substr($content, $imgBeg);
        $imgEnd = strpos($post, '>');
        $postOutput = substr($post, 0, $imgEnd+1);
        $postOutput = preg_replace('/width="([0-9]*)" height="([0-9]*)"https://wordpress.stackexchange.com/",
                                   '',
                                   $postOutput);
        $postOutput = str_replace(array('class="alignright"', 'class="alignleft"'), '', $postOutput);
        $postOutput = preg_replace('/title="([\w-_]+)"https://wordpress.stackexchange.com/",
                                   '',
                                   $postOutput);
        $image[$i] = $postOutput;
        $start=$imgEnd+1;
    }

    if(isset($image) && stristr($image[$num],'<img')) {
        return str_replace('<img ', '<img ' . image_hwstring($width, $height), $image[$num]);
    }
}

function get_post_attachment_thumbnail($width="", $height="") {
    $files = get_children('post_parent=".get_the_ID()."&post_type=attachment&post_mime_type=image');
    if($files) {
        $keys = array_reverse(array_keys($files));
        $j=0;
        $attachment_id = $keys[$j];
        $image = image_downsize($attachment_id, 'thumbnail');
        $image[0] = str_replace('http://yoast.com', 'http://cdn.yoast.com', $image[0]);
        return '<img src="' . $image[0] . '" alt="" ' . image_hwstring($width, $height) . '/>';
    }
}

function get_post_category_image($p_id=null, $width="", $height=""){
    global $post_id, $top_category;
    if (!$p_id)
        $p_id = $post_id;

    if ( isset($top_category) )
        return $top_category;

    $post_categories = get_the_category( $p_id );

    $slug = count($post_categories) ? $post_categories[0]->slug : '';

    if ($slug && file_exists(TEMPLATEPATH . '/img/post-img/'.$slug.'.png')){
        return '<img ' . image_hwstring($width, $height). ' src="' . get_bloginfo('template_url') . '/img/post-img/'.$slug.'.png" alt="'.$post_categories[0]->cat_name.'"/>';
    } else {
        return '<img ' . image_hwstring($width, $height). ' src="' . get_bloginfo('template_url') . '/img/post-img/default.jpg" alt=""/>';
    }
}

function get_post_image($string_size="post-thumbnail", $width=0){
    if ( has_post_thumbnail() ) :
        return get_the_post_thumbnail(NULL, $string_size, array('title'=>''));
    elseif( $attached_image = get_post_attachment_thumbnail($width)):
        return $attached_image;
    elseif( $post_content_thumbnail = get_post_content_thumbnail(1, $width)):
        return $post_content_thumbnail;
    else:
        return get_post_category_image(NULL, $width);
    endif;
}