jquery form plugin in wordpress to upload an image file does not work as expected

jquery form plugin in wordpress to upload an image file does not work as
expected

Trying to use the jquery form plugin in a worpress site .
HTML:
<form id="imageform" method="post" action="<?php
bloginfo('template_directory')?>/ajaximage.php "
enctype="multipart/form-data" accept-charset="utf-8" >
Upload image:
<input type="file" name="photoimg" id="photoimg" value="" />
</form>
<button value="&nbsp;Upload&nbsp;"
class="btn_upload">&nbsp;Upload&nbsp;</button>
<div id='preview'></div><!-- end of id preview-->
JS:
$(document).ready(function(){
$(".btn_upload").click(function(){
$("#preview").html('');
$(".result_upload").html('<img src="'+loc+'/images/ajax-loader.gif"
alt="wait.."/>');
$("#imageform").ajaxForm({
target: '#preview'
}).submit();
});
});
ajaximage.php has
$photoimg = trim($_POST['photoimg']);
echo 'got it';
If I do not select any file , I get the result as expected. But if I
select an image file then the error message says - "Undefined index:
photoimg in line ...."
How to make it work correctly?