<?
$imagens = array("Figura1.gif","figura2.gif","figura3.gif");
$url = array("url1","url2","url3");

srand((double)microtime()*1000000);
$select=rand(3,-1);

echo "<a href='".$url[$select]."' target='_blank'><img src='".$imagens[$select]."' border=0></a>";
?>