bbPress.org

function get_post_img(){

global $bbdb;

$topic_id_ft = get_topic_id(); //topic id for getting text of first post of the topic

$first_post = (int) $bbdb->get_var("SELECT post_id FROM $bbdb->posts WHERE topic_id = $topic_id_ft ORDER BY post_id ASC LIMIT 1");

$content = substr(strip_tags(strip_shortcodes(get_post_text($first_post))),0);

preg_match('/[img](.*?)[/img]/i', $content, $matches);

$src = $matches[1];

echo $src;

}

modified from here http://bbpress.org/forums/topic/show-a-snippet-of-the-post-under-the-topic-title#post-39260