Fix en función de Javascript

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>pruebas javascript</title>
</head>

<body>
<script>
var imagenes = new Array();
    imagenes[0] = "001.jpg";
    imagenes[1] = "002.jpg";
    imagenes[2] = "003.jpg";
    imagenes[3] = "004.jpg";
    imagenes[4] = "005.jpg";
    imagenes[5] = "006.jpg";

    indice=0;

function mas(){
    indice+=1;
    if(indice>5){
        indice=0;
        }
    document.images["imagen"].src = imagenes[indice];
    alert("test");
}
function menos(){
    indice-=1;
    if (indice<0){
        indice=5;
    }
    document.images["imagen"].src=imagenes[indice];
}
</script>
<table border="0">
    <tr>
        <td colspan="2">
        <img src="001.jpg" name="imagen">
        </td>
    </tr>
    <tr>
        <td>
            <button onclick="menos();">-</button>
        </td>
        <td>
            <button onclick="mas();">+</button>
        </td>
     </tr>
 </table>


</body>
</html>