<?php $krsm = get_post_meta($post->ID,'thumbnail', true);
if($krsm != "") { ?>
<a href="<?php the_permalink() ?>">
<img src="<? echo strtolower($krsm); ?>"/></a>
<?php }else{ ?>
<a href="<?php the_permalink() ?>"><img src="http://www.siten.com/resimyok.jpg" alt="" /></a>