var clickmessage = "Our images are protected"; 
function disableclick(e)
{
    if (document.all)
    {
        if (event.button == 2 || event.button == 3)
        {
            if (event.srcElement.tagName == "IMG")
            {
                alert(clickmessage);
                return false;
            }
        }
    }
    else if (document.layers)
    {
        if (e.which == 3)
        {
            alert(clickmessage);
            return false;
        }
    }
    else if (document.getElementById)
    {
        if (e.which == 3 && e.target.tagName == "IMG")
        {
            alert(clickmessage);
            return false;
        }
    }
}
function associateimages()
{
    for (i = 0; i < document.images.length; i++)
        document.images[i].onmousedown = disableclick;
}
if (document.all)
    document.onmousedown = disableclick;
else if (document.getElementById)
    document.onmouseup=disableclick;
else if (document.layers)
    associateimages();