probabilmente così:
Codice PHP:
<script language="javascript" type="text/javascript">
<!--
 var winheight=100;
 var winwidth=100;
 var cat=null;

 step=1;

 var nav=navigator.userAgent.toLowerCase();
 var isopera=false;
 if (nav.indexOf('opera')!=-1) isopera=true

function catswindows(imgname,imgwidth,imgheight,imgtitle) 
{
 if (isopera)
  {
  LeftPosition=(screen.width)?(screen.width-winwidth)/2:100;
  TopPosition=(screen.height)?(screen.height-winheight)/2:100;
  paramstp="height="+winheight+",width="+winwidth+",top="+TopPosition+
  ",left="+LeftPosition+",scrollbars=no,location=no"+
  ",directories=no,status=no,menubar=no,toolbar=no,resizable=no";

  var operastr='<html><title>'+imgtitle+'</title><body>[img]'+imgname+'[/img]</b'+'ody></h'+'tml>';
  cat=window.open("blank4opera.html","cat",paramstp);
  cat.document.write(operastr);

  x=y=step;
  while (x | y)
   {
    cat.resizeBy(x,y);
    LeftPosition=(screen.width-cat.window.innerWidth)/2;
    TopPosition=(screen.height-cat.window.innerHeight)/2;
    if (LeftPosition<0) LeftPosition=0;
    if (TopPosition<0) TopPosition=0;
    cat.moveTo(LeftPosition,TopPosition);
    if (cat.window.innerWidth>=imgwidth) x=0;
    if (cat.window.innerHeight>=imgheight) y=0;
   }
 
  if(cat.focus){cat.focus();} 
  return;
  } 

 if (!document.layers&&!document.all&&!document.getElementById)
  {
   paramstp="height="+imgheight+",width="+imgwidth+",top=6"+
   ",left=6,scrollbars=no,location=no"+
   ",directories=no,status=no,menubar=no,toolbar=no,resizable=no"
   cat=window.open("","cat",paramstp);
   oldstr='<html><title>'+imgtitle+'</title><body background="'+imgname+'"></b'+'ody></h'+'tml>';
   cat.document.write(oldstr);
   if (cat.focus){cat.focus();}
   return;
  }
 
 if (document.layers)
  {
   LeftPosition=(screen.width)?(screen.width-imgwidth)/2:100;
   TopPosition=(screen.height)?(screen.height-imgheight)/2:100;
   paramstp="height="+imgheight+",width="+imgwidth+",top="+TopPosition+
   ",left="+LeftPosition+",scrollbars=no,location=no"+
   ",directories=no,status=no,menubar=no,toolbar=no,resizable=no"}
 else
  {
  LeftPosition=(screen.width)?(screen.width-winwidth)/2:100;
  TopPosition=(screen.height)?(screen.height-winheight)/2:100;
  paramstp="height="+winheight+",width="+winwidth+",top="+TopPosition+
  ",left="+LeftPosition+",scrollbars=no,location=no"+
  ",directories=no,status=no,menubar=no,toolbar=no,resizable=no"}

 cat=window.open("","cat",paramstp);

 if (document.all)
  {
   string="<html><title>"+imgtitle+"</title><body topmargin=0 leftmargin=0 bgcolor=black>";
   string1="<div id=loadintxt style='position:absolute;width:100%;top:45%;text-align:center;visibility:visible;font-family:Comic Sans MS;font-size:16px;font-weight:bold'>Loading...</div>";
   string2="<div id=errtxt style='position:absolute;width:100%;top:45%;text-align:center;font-family:Comic Sans MS;font-size:16px;color:red;font-weight:bold;visibility:hidden'>ERROR!!! Please try again</div>";
   string3='<s'+'cript>'+'var animColorIndex=0;colorArray=["#FFFFFF","#CCCCCC","#999999","#666666","#333333","#000000","#333333","#666666","#999999","#CCCCCC"];';
   string4='function textLooping(){if(animColorIndex==10)animColorIndex=0;'+
   'loadintxt.style.color=colorArray[animColorIndex];animColorIndex++;}animInterval=setInterval("textLooping()",200);';
   string5="function erroccur(){clearInterval(animInterval);loadintxt.style.visibility='hidden';errtxt.style.visibility='visible';}";
   string6="</s"+"cript>";
   string7='<s'+'cript language="jav'+'ascript1.2">';
   string8='function init(){clearInterval(animInterval);'+
   "loadintxt.style.visibility='hidden';"+
   'fltr.filters.item(0).Apply();fltr.filters.item(0).Play();'+
   'fltr.style.visibility="visible";}'+'</s'+'cript>';
   string9='[img]'+imgname+'[/img]'+
   '</b'+'ody></h'+'tml>';
  } 
 
 if (document.layers)
  {
   toppos=imgheight/2-40;
   leftpos=imgwidth/2-40;
   string="<html><title>"+imgtitle+"</title><body bgcolor='#000000'><div id=loadintxt style='position:absolute;top:"+toppos+"px;left:"+leftpos+"px;'></div>"
   string1='<s'+'cript>var animColorIndex=0;colorArray=["#FFFFFF","#CCCCCC","#999999","#666666","#333333","#000000","#333333","#666666","#999999","#CCCCCC"];'
   string2='function textLooping(){if(animColorIndex==10)animColorIndex=0;document.loadintxt.document.open();'
   string3='document.loadintxt.document.write("<center><font size=4 face=Comic Sans MS color="+colorArray[animColorIndex]+">[b]Loading...[/b]</font></center>");document.loadintxt.document.close();'
   string4='animColorIndex++;};animInterval=setInterval("textLooping()",200);</s'+'cript>'
   string5='<s'+'cript language="jav'+'ascript1.2">'
   string6="function init(){document.catpic.visibility='show';clearInterval(animInterval);document.loadintxt.visibility='hide';};"
   string7="function erroccur(){errstr='ERROR!!! Please try again';alert(errstr);}</s"+"cript>"
   string8="<div id=catpic style='position:absolute;left:0px;top:0px;visibility:hidden'>"
   string9="[img]"+imgname+"[/img]</div></b"+"ody></h"+"tml>";
  }
 
 if (document.getElementById&&!document.all)
  {
   toppos=imgheight/2-40;
   leftpos=imgwidth/2-40;
   string="<html><title>"+imgtitle+"</title><body bgcolor='#000000'><div id=loadintxt style='position:absolute;top:"+toppos+"px;left:"+leftpos+"px;'></div>"
   string1='<s'+'cript>var animColorIndex=0;colorArray=["#FFFFFF","#CCCCCC","#999999","#666666","#333333","#000000","#333333","#666666","#999999","#CCCCCC"];';
   string2='function textLooping(){if(animColorIndex==10)animColorIndex=0;ls="<font size=4 face=Comic Sans MS color="+colorArray[animColorIndex]+">[b]Loading...[/b]</font>"';
   string3='document.getElementById("loadintxt").innerHTML=ls;'
   string4='animColorIndex++;};animInterval=setInterval("textLooping()",200);</s'+'cript>'
   string5='<s'+'cript language="jav'+'ascript1.2">'
   string6='function init(){document.getElementById("catpic").style.visibility="visible";clearInterval(animInterval);document.getElementById("loadintxt").style.visibility="hide";}';
   string7="function erroccur(){errstr='ERROR!!! Please try again';alert(errstr);}</s"+"cript>"
   string8="<div id=catpic style='position:absolute;left:0px;top:0px;visibility:hidden'>";
   string9="[img]"+imgname+"[/img]</div></b"+"ody></h"+"tml>";
  }

 cat.document.writeln(string);
 cat.document.writeln(string1);
 cat.document.writeln(string2);
 cat.document.writeln(string3);
 cat.document.writeln(string4);
 cat.document.writeln(string5);
 cat.document.writeln(string6);
 cat.document.writeln(string7);
 cat.document.writeln(string8);
 cat.document.writeln(string9);

 x=y=step;

 if (document.all)
  {
   while (x | y)
   {
    cat.resizeBy(x,y);
    LeftPosition=(screen.width-cat.document.body.clientWidth)/2;
    TopPosition=(screen.height-cat.document.body.clientHeight)/2;
    if (LeftPosition<0) LeftPosition=0;
    if (TopPosition<0) TopPosition=0;
    cat.moveTo(LeftPosition,TopPosition);
    if (cat.document.body.clientWidth>=imgwidth) x=0;
    if (cat.document.body.clientHeight>=imgheight) y=0;
   }
  }

 else
  if (document.getElementById&&!document.all)
  {
   while (x | y)
   {
    cat.resizeBy(x,y);
    LeftPosition=(screen.width-cat.window.innerWidth)/2;
    TopPosition=(screen.height-cat.window.innerHeight)/2;
    if (LeftPosition<0) LeftPosition=0;
    if (TopPosition<0) TopPosition=0;
    cat.moveTo(LeftPosition,TopPosition);
    if (cat.window.innerWidth>=imgwidth) x=0;
    if (cat.window.innerHeight>=imgheight) y=0;
   }
  }
 
 if(cat.focus){cat.focus();} 
 return;
}

function CloseCatWin()
 {if(cat!=null && cat.open)cat.close()}
 
window.onfocus=CloseCatWin;

//-->
</script> 
Ma è brutto assai come effetto, lightbox è molto meglio!