coordinate posozione watermark

[horusbird]

Ho un codice che crea l'effetto watermark su delle immagini di un forum, attualmente il watermark (una scritta su un rettangolo semiopaco) viene posizionata in alto e in basso.
Vorrei invece che l'immagine venisse invece posizionata esattamente al centro sia in verticale che in orizzontale, qualcuno pratico mi darebbe una mano?

Grazie mille :)

ecco il codice:

$tempimage='';
$referers_count=0;
$referers=array();
if($vbulletin->options['watermark_referer']) {
    $referers=explode(',',$vbulletin->options['watermark_referer']);
    if(empty($_SERVER['HTTP_REFERER'])) {
        $referers_count=1;
    }
    else {
        str_replace($referers,'',$_SERVER['HTTP_REFERER'],$referers_count);
    }
}

if(!isset($_SERVER['HTTP_RANGE'])
    &&in_array($extension,array('jpg','png','gif'))
    &&$attachmentinfo['dateline']<TIMENOW-$vbulletin->options['watermark_days']*86400
    &&(!$referers||($referers&&!$referers_count)))
{
    if($vbulletin->options['watermark_thumb']&&empty($vbulletin->GPC['thumb'])) {
        $clearfname = DIR.'/'.$vbulletin->options['cleargifurl'];
        if( file_exists($clearfname) && $fp2 = fopen($clearfname,'rb') ) {
            header('Content-Length: '. filesize($clearfname));
            header('Content-type: image/gif');
            @fclose($fp);
            $fp = $fp2;    
            $lastbyte = filesize($clearfname)-1;    
        }        
    }    else {        
    
        $wm_t = @imagecreatefrompng($vbulletin->options['watermark_path_top']);
        $wm_b = @imagecreatefrompng($vbulletin->options['watermark_path_bottom']);
                    
        if (strtolower($extension) == "gif") {
            $im_a = @imagecreatefromgif($attachpath);
        }
        else if(strtolower($extension) == "jpg") {
            $im_a = @imagecreatefromjpeg($attachpath);
        } 
        else if(strtolower($extension) == "png") {
            $im_a = @imagecreatefrompng($attachpath);
        }
    
        if($im_a) {
            $tempimage = @imagecreatetruecolor(imagesx($im_a), imagesy($im_a));
            imagecopy($tempimage, $im_a,    0, 0, 0, 0, imagesx($im_a), imagesy($im_a));
            $im_a = $tempimage;    
    
            if($wm_t&&imagesx($im_a)>imagesx($wm_t)) {    
                imagefilledrectangle($im_a, 0 , 0 , imagesx($im_a) , imagesy($wm_t) , imagecolorallocatealpha($im_a, 0, 0, 0, 100) );
                imagecopy($im_a, $wm_t, imagesx($im_a)-imagesx($wm_t), 0, 0, 0, imagesx($wm_t), imagesy($wm_t));
            }
            if($wm_b&&imagesx($im_a)>imagesx($wm_b)) {    
                imagefilledrectangle($im_a, 0 , imagesy($im_a)-imagesy($wm_b) , imagesx($im_a) , imagesy($im_a) , imagecolorallocatealpha($im_a, 0, 0, 0, 100) );
                imagecopy($im_a, $wm_b, imagesx($im_a)-imagesx($wm_b), imagesy($im_a)-imagesy($wm_b), 0, 0, imagesx($wm_b), imagesy($wm_b));
    
            }
    
            $tmpfname = tempnam($vbulletin->options['tmppath'],'watermark_');
            
            if (strtolower($extension) == "gif") {
                imagegif($im_a,$tmpfname);
            }
            else if(strtolower($extension) == "jpg") {
                imagejpeg($im_a,$tmpfname);
            } 
            else if(strtolower($extension) == "png") {
                imagepng($im_a,$tmpfname);
            }        
                
            if($wm_t) imagedestroy($wm_t);
            if($wm_b) imagedestroy($wm_b);
            imagedestroy($im_a);
    
            if( file_exists($tmpfname) && $fp2 = fopen($tmpfname,'rb') ) {
                header('Content-Length: '. filesize($tmpfname));
                @fclose($fp);
                $fp = $fp2;    
                $lastbyte = filesize($tmpfname)-1;    
            }
        }
    }
} 

ed ecco come appare il watermark: