window.onload=function()
 {
  var
  base='../wp-content/themes/organic-theme/img/logos/',
  current = 'Default',
  thumb=document.getElementById('thumb'),
  sel=document.getElementById('organic_theme_logo');
  if(sel != null) {
  sel.onchange=function()
  {
  thumb.src=base+this.value+'.gif';
  }
  thumb.src=base+current+'.gif';
}
   }