bbPress.org

I think this does what you want

add_filter ('bbp_get_reply_author_avatar', 'rew_avatar', 10 , 3) ;
add_filter ('bbp_get_topic_author_avatar', 'rew_avatar' , 10 , 3) ;

function rew_avatar ( $author_avatar, $id, $size)  {
	if ( bbp_is_reply( $id ) ) {
		$author = bbp_get_reply_author_display_name( $id );
		$text = 'alt="icon avatar for '.$author.'"' ; 
	}
	if ( bbp_is_topic( $id ) ) {
		$author = bbp_get_topic_author_display_name( $id );
		$text = 'alt="icon avatar for '.$author.'"' ; 
	}
	$author_avatar = str_replace('alt=\'\'',$text,$author_avatar);
return $author_avatar ;
}