<?php if( get_post_meta($post->ID, "resim", true) ): ?>
<img src="<?php $values = get_post_custom_values("resim"); echo $values[0]; ?>" alt="<?php the_title(); ?>" />
<?php else: ?>
<img src="<?php bloginfo('template_directory'); ?>/resimyok.jpg" alt="<?php the_title(); ?>" />
<?php endif; ?>