var prefix = "images/edgettimage";
var image = new Array(8);
var i = 0;

for (var i=0; i<image.length; i++)
{
    image[i] = prefix + (i+1) + ".jpg";
}

i = 0;

function rotate()
{
    i++;
    if (i >= image.length)
    {
        i = 0;
    }
    document.getElementById('placeholder').src=image[i];
}

function startRotate()
{
    document.getElementById('placeholder').src=image[0];
    setInterval('rotate()', 5000);
}