example of image widget using P&T assets, you'll need to update the ACT id for outputting the image thumb, relevant to your install.

gistfile1.html

<table class="mainTable entry_widget_notop" border="0" cellspacing="0" cellpadding="0" data-index="0"> 
	<tbody>
		<tr>
			<td style="width: 100px;">

				<?php if($options['image_id']):?>
					<img src="/?ACT=40&file_id=<?=$options['image_id']?>&size=100x100&hash=1361909881">
				<?php endif ?>

				<p><a href="#" class="thumbnail_trigger">Add File</a></p>
				<div class="filename_holder" style="display:none;">
				<?php echo form_hidden($field_name.'[image_id]', $options['image_id']); ?>
				</div>

				

			</td>
			<td><label>Title</label>
				<?php echo form_input($field_name.'[image_title]', $options['image_title']); ?>
			</td>
			<td style="width: 100px;"><label>Style</label>
				<?php echo form_dropdown($field_name.'[image_type]', $options['image_type_options'], $options['image_type']); ?>
			</td>
		</tr>
	</tbody>
</table>



<script type="text/javascript">
$(document).ready(function(){

	$('.thumbnail_trigger').click( function(e) {
		e.preventDefault();

		var td = $(this).closest("td");

		var mySheet = new Assets.Sheet({

		    // optional settings (these are the default values):
		    multiSelect: false,
		    filedirs:    'all', // or array of filedir IDs
		    kinds:       ['image'], // or array of file kinds ("image", "flash", etc)

		    // onSelect callback (required):
		    onSelect:    function(files) {
		        console.log(files);
		       	td.find(".filename_holder input").val(files[0].id);
		       	td.find('img').remove();
		       	td.prepend('<img src="/?ACT=40&file_id='+files[0].id+'&size=100x100&hash=1361909881">')
		    }
		});
		
		mySheet.show();

	});

});

</script>

To get the thumbnail url code like this can be used:

var thumb_url = Assets.siteUrl + '?ACT=' + Assets.actions.view_thumbnail+'&file_id='+file_id+'&size=50x50&hash='+Math.random();