upload image and set it as background

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Javascript

CSS

HTML

PHP

jQuery

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hey,

i want my users on my website to be able to change the bodys background image via

Background:

in css.

so far i have:

<input type="file" id="file_upload" name="file_upload" />
   <a href="java<!-- no -->script:$('#file_upload').uploadifyUpload();">Upload File</a>

and the jQuery

$(document).ready(function() {
         $('#file_upload').uploadify({
      'uploader'    : '/uploadify/uploadify.swf',
      'script'      : '/uploadify/uploadify.php',
      'cancelImg'   : '/uploadify/cancel.png',
      'folder'      : '/uploads',
	  'sizeLimit'   : 204800,
	  'onComplete'  : function(event, ID, fileObj, response, data) {
      alert('There are ' + data.fileCount + ' files remaining in the queue.');
    }
    });
    });

i want to replace the 'alert' with a string which could do this...
any ideas on what to do next?

p.s
My server supports PHP