Réponse acceptée !
<script language="JavaScript">
function PopupImage(img) {
w=open("",'chargement','width=1,height=1,toolbar=no,scrollbars=no,resizable=no');
w.document.open();
w.document.write("<HTML><HEAD><TITLE>"+img+"</TITLE></HEAD>");
w.document.write("<SCR"+"IPT language=javascript>function checksize() { if (document.images[0].complete) { window.resizeTo(document.images[0].width,document.images[0].height);
moveTo( ( screen.width-document.images[0].width)/2 , ( screen.height-document.images[0].height)/2); window.focus();} else { setTimeout('checksize()',250) } } </SC"+"RIPT>");
w.document.write("<BODY onload='checksize()' onclick='window.close()' leftMargin=0 topMargin=0 marginwidth=0 marginheight=0>");
w.document.write("<center><IMG src='"+img+"' border=0 ></center>");
w.document.write("</BODY></HTML>");
w.document.close();
}