.logo_box {
    position: absolute;
    top: 0px;
    left: 0px;
    height: 65px;
  }

  #client_logos {
    position: absolute;
    left: 100px;
    top: 20px;
    margin: auto;
    width: 646px;
    height: 50px;
    overflow: hidden;
  }

function startLeftScroll(id){
  var box = document.getElementById(id);
  var box2 = document.createElement('div');
  box2.id = id+'_virtual';
  box2.innerHTML = box.innerHTML;
  box2.className = 'logo_box';
  box.parentNode.appendChild(box2);
  moveABit(id,0);
}

function moveABit(id,position){
    var keepscrolling = true;
    var box = document.getElementById(id);
    var other_box = document.getElementById(id+'_virtual');
    var width = box.offsetWidth;
    position = position - 1;
    box.style.left = position+'px';
    var other_position = position + width;
    other_box.style.left = other_position +'px';
     // if main box is completely hidden, move behind virtual
    var diff = (width) + position;

    if ( diff <= 1 ){
      position = 0;
    }
    if ( keepscrolling )
      setTimeout("moveABit('"+id+"','"+position+"');",100);
}

<div id="client_logos">
<div style="position:absolute;width:9000px;">
<div id="logo_box" class="logo_box">

<img src="Image1">
<img src="Image2">
<img src="Image3">  

</div>
</div>
</div>