image_preview.js

var _ImagePreview = function() {
  this.image = null;
  this.image_object = new Image();
  this.image_view = function(el,dest)
  {
    var imagefile = document.getElementById(el);
    this.image = document.getElementById(dest);
    // HTML5 FileAPI: Firefox 3.6+, Chrome 6+
    if(typeof(FileReader)!='undefined')
    {
      var reader = new FileReader();
      var self = this;
      reader.onload = function(e){
        self.image.src = e.target.result;
      }
      reader.readAsDataURL(imagefile.files.item(0));
    }
    // Firefox 3.0-3.6
    else if(imagefile.files && imagefile.files.item(0).getAsDataURL)
    {
      this.image.src = imagefile.files.item(0).getAsDataURL();
    }
    // Safari
    else if(imagefile.files && !imagefile.files.item(0).getAsDataURL)
    {
      //Safari and webkit not supported now.
    }
    //IE!!!!
    else
    {
      try
      {
        this.image_object.src = imagefile.value;
        var self = this;
        setTimeout(function(){
          self.image.src = self.image_object.src;
          },200);
      }
      catch(e){}
    }
  }	  
}
var ImagePreview = new _ImagePreview();

I wrote a slightly more generalized version that'll push you a URL whenever one changes, so you can use it for more cases.