Only show image from custom field when present

by 

<?php
    $imageid = get_post_meta($post->ID, 'imageembed', true);
    if($imageid != '') 
       { ?>
        <div class="newsimage"><img src="https://wordpress.stackexchange.com/questions/57967/<?php echo wp_get_attachment_url( $imageid ); ?>" width="640" /></div>
<?php  } ?>

Use if condition to check if $imageid has any value in it or not.