http://forum.pluxml.org/viewtopic.php?id=6291

plxShow.php

<?php
/* --------- plxShow::artThumbnail() ---------- */
public function artThumbnail($format='<a href="#img_url"><img class="art_thumbnail" src="#img_thumb_url" alt="#img_alt" title="#img_title" /></a>', $echo=true) {

    $filename = trim($this->plxMotor->plxRecord_arts->f('thumbnail'));
    if(!empty($filename)) {
        $img_url = $this->plxMotor->urlRewrite($filename);
        $img_thumb = plxUtils::thumbName($filename);
        $replaces = array(
            '#img_url'      => $img_url,
            '#img_thumb_url'=> (file_exists(PLX_ROOT.$img_thumb)) ? $this->plxMotor->urlRewrite($img_thumb) : $img_url,
            '#img_title'    => plxUtils::strCheck($this->plxMotor->plxRecord_arts->f('thumbnail_title')),
            '#img_alt'      => $this->plxMotor->plxRecord_arts->f('thumbnail_alt')
        );
        if($echo)
            echo str_replace(array_keys($replaces), array_values($replaces), $format);
        else
            return str_replace(array_keys($replaces), array_values($replaces), $format);
    } elseif(!$echo)  {
        return false;
    }

}
?>

plxUils.php

<?php
/* -------- plxUtils::thumbName($filename) -------- */
	public static function thumbName($filename) {
		if(preg_match('/^(.*\.)(jpe?g|png|gif)$/iD', $filename, $matches)) {
			return $matches[1].'tb.'.$matches[2];
		} else {
			return $filename;
		}
	}

?>