preview image pics before upload javascript

<script type="text/javascript">
function showImage(str)
{
    document.body.innerHTML+='<br>Uploaded image path: <samp>'+str+'</samp><br><img src="'+str+'" height="100" width="150" alt="">';
}
</script>
<input type="file" id="MainImage" onChange="showImage(this.value)">

<script type="text/javascript">

var maxWidth=100;
  // height to resize large images to
var maxHeight=100;
  // valid file types
var fileTypes=["bmp","gif","png","jpg","jpeg"];
  // the id of the preview image tag
var outImage="previewField";
  // what to display when the image is not valid
var defaultPic="spacer.gif";

/***** DO NOT EDIT BELOW *****/

function preview(what){
  var source=what.value;
  var ext=source.substring(source.lastIndexOf(".")+1,source.length).toLowerCase();
  for (var i=0; i<fileTypes.length; i++) if (fileTypes[i]==ext) break;
  globalPic=new Image();
  if (i<fileTypes.length) globalPic.src=source;
  else {
    globalPic.src=defaultPic;
    alert("THAT IS NOT A VALID IMAGE\nPlease load an image with an extention of one of the following:\n\n"+fileTypes.join(", "));
  }
  setTimeout("applyChanges()",200);
}
var globalPic;
function applyChanges(){
  var field=document.getElementById(outImage);
  var x=parseInt(globalPic.width);
  var y=parseInt(globalPic.height);
  if (x>maxWidth) {
    y*=maxWidth/x;
    x=maxWidth;
  }
  if (y>maxHeight) {
    x*=maxHeight/y;
    y=maxHeight;
  }
  field.style.display=(x<1 || y<1)?"none":"";
  field.src=globalPic.src;
  field.width=x;
  field.height=y;
}
// End -->
</script>

<center> Pre-veiw pic from your drive </center>
<div align="center" style="line-height: 1.9em;">

<br>
<input name="userfile" type="file" id="picField" onchange="preview(this)" >

      <br><img id="previewField" alt="" name="previewField" width="1" border="2" height="1" >