That is why I suggest you an addon. I.e.:
<?php
/*
Requires Pixelpost version 1.5 or newer
GeoS ADDON-Version 0.1
Written by: Piotr "GeoS" Galas
@: piotr.galas+pixelpost@gmail.com
WWW: http://blog.piotrgalas.com/
Pixelpost www: http://www.pixelpost.org/
License: http://www.gnu.org/copyleft/gpl.html
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License
as published by the Free Software Foundation; either version 2
of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*/
$addon_name = "GeoS Template ADDON";
$addon_description = "GeoS Template ADDON give new tags for template use.<p />";
$addon_version = "1";
/*****************/
/* CODE of ADDON */
/*****************/
$template_nav_width = $image_width / 2;
$tpl = ereg_replace("<GEOS_NAV_WIDTH>","$template_nav_width",$tpl);
?>
And thanks to this I use tag <GEOS_NAV_WIDTH> as width of layer with transparent image which is link to next/prev image.
PS Remember to add px after tag - <GEOS_NAV_WIDTH>px in styles definition.