<script type="text/javascript" language="JavaScript">
var now = new Date ();
var day = now.getDay();
//RECUERDA- LA SEMANA EMPIEZA EN DOMINGO
if(day == 0) document.write("<img src='http://www.URL de la imagen.com' alt='domingo'>");
if(day == 1) document.write("<img src='http://www.URL de la imagen.com' alt='lunes'>");
if(day == 2) document.write("<img src='http://www.URL de la imagen.com' alt='martes'>");
if(day == 3) document.write("<img src='http://www.URL de la imagen.com' alt='miercoles'>");
if(day == 4) document.write("<img src='http://www.URL de la imagen.com' alt='jueves'>");
if(day == 5) document.write("<img src='http://www.URL de la imagen.com' alt='viernes'>");
if(day == 6) document.write("<img src='http://www.URL de la imagen.com' alt='sabado'>");
</script>