BuddyPress.org

Try this:

function wpse_49216_my_new_avatar_url() {
global $bp;
	if( bp_is_user() && !bp_get_member_user_id() ) {
        $user_id = 'displayed: '. bp_displayed_user_id();
    } else {
        $user_id = 'get_member_user: '. bp_get_member_user_id();
    }

$gender = xprofile_get_field_data ( 'ranking', $user_id );
   if ($gender == "Top 25") {
      return 'http://hsseek2.esy.es/wp-content/uploads/2015/04/25.png';
   }
   if ($gender == "Top 24") {
      return 'http://hsseek2.esy.es/wp-content/uploads/2015/04/24.png';
   }
}
add_filter ('bp_core_fetch_avatar_url', 'wpse_49216_my_new_avatar_url');