un esempio:

html/css
codice:
#image{
  position:absolute;
  top:10px;
  left:10px;
}

[img]oconnor.jpg[/img]
js
codice:
var x = 10;
var y = 10;
var move;

function moveImage()
{
document.getElementById("image").style.top = ++y + "px";
document.getElementById("image").style.left = ++x + "px";
move = setTimeout(moveImage,100);
}

function stopImage()
{
clearInterval(move);
}