[ZoNT]

function Mausklick (e) {
  if(flag != null) {
  if (document.getElementById('sklalDiv')) {
    var d = document.getElementById("test123");
    d.style.width = e.clientX - d.offsetLeft + "px" ;
    d.style.height = e.clientY - d.offsetTop + "px" ;
    document.getElementById("ausgabe").innerHTML = e.clientX + "px" + e.clientY + "px";
  }
  }
}

function ausfuehren (e) {
  e = e||window.event;
  if (e.preventDefault) e.preventDefault();
  else e.returnValue = false;
  document.onmousemove = Mausklick;
  flag = 1;
  document.onmouseup = Stop;
}

<img src="status_rot.gif" alt="" onmousedown="ausfuehren(event);" />