function get_first_image() {
    global $post;
    $PostID = $post->ID;

    $all_images =& get_children('post_type=attachment&post_mime_type=image&post_parent=' . $PostID );

    if($all_images) {
        $arr_of_all_images = array_keys($all_images);

        // Get the first image attachment from post
        $firstImage = $arr_of_all_images[0];

        // Get the thumbnail url for the attachment
        // If you want the full-size image instead of the thumbnail, use wp_get_attachment_url() instead of wp_get_attachment_thumb_url().
        $thumb_url = wp_get_attachment_thumb_url($firstImage);

        echo $thumb_url;
    }
}