<h1><?php the_title(); ?></h1>
<?php $resim = get_post_meta($post->ID, 'resim', true);
 if($resim) : ?>
<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?><img class="alignleft" src="<?php echo $resim; ?>" width="615" height="315" alt="<?php the_title(); ?>" /></a>
<?php else : ?>
<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?><img class="alignleft" src="resim-urlsi-buraya" width="615" height="315" alt="<?php the_title(); ?>" /></a>
<?php endif; ?>