<script type="text/javascript">
function click (e) {
if (!e)
e = window.event;
if ((e.type && e.type == "contextmenu") || (e.button && e.button == 2) || (e.which && e.which == 3)) {
if (window.opera)
window.alert("");
return false;
}
}
if (document.layers)
document.captureEvents(Event.MOUSEDOWN);
document.onmousedown = click;
document.oncontextmenu = click;
</script>
I have found this script on selfhtml.org.
This function is originally designed to disable the client side context menu and to insert your own context menu. But it can be used for this too.
But keep in mind: By using browser addons like NoScript or opening the image url user could get around this.