Hi,
the first thing you need to look at is the [url=http://www.phpbb.com/mods/documentation/phpbb-documentation/dbal/index.php]dbal[/url]. Your current code is only mysql compatible. You need to use the $db->mysql_query_limit() function/method.
Secondly, have a look at the [url=http://www.phpbb.com/mods/documentation/phpbb-documentation/template/index.php]templates system[/url]. You have to use $template->assign_block_vars instead of the <!-- PHP -->.
Thirdly, use $phpbb_root_path, i don't think this is very good for use on many differen't servers: http://www.hardheavy.sk/gallery/thumbnail.php?pic_id=$id
Another thing is, you should use table constants. Look in includes/constants.php, and scroll down, then you'll know what i mean.