<?php $resim = get_post_meta($post->ID,'Thumbnail', true); 
if($resim != "") {  ?>
<a href="<?php the_permalink() ?>"><img style="padding:1px; margin:0px; border:0px;" width="80" height="80" src="<? echo strtolower($resim); ?>"></a>
<?php }else{ ?>
<a href="<?php the_permalink() ?>"><img style="padding:1px; margin:0px; border:0px;" width="80" height="80" src="http://www.siteniz.com/defaultresim.jpg"></a>
<?php } ?>