while(true){
aleatorio = Math.random();
numero = Math.round(aleatorio * 4);

if (numero == 1){
imagen = "http://img260.imageshack.us/img260/3114/maflandreborn27.png";
break;}

if (numero == 2){
imagen = "http://img260.imageshack.us/img260/9762/maflandsqualo1.png";
break;}

if (numero == 3){
imagen = "http://img80.imageshack.us/img80/6870/maflandcopia.png";
break;}

if (numero == 4){
imagen = "http://img260.imageshack.us/img260/4382/maflandlambo.png";
break;}

}
document.write('<a href="url del foro" target="_self" ><img src="' + imagen + '" border=0 style="width: 650px; height: 180px"></a>');