Custom Field is header pic, limit to one per page?

I’m doing a CMS WP site and want each page to be able to have a different picture in the header.

I’ve added a custom field called picture and this code to the header:

<?php if (have_posts()) : while (have_posts()) : the_post();
		$picturevalues = get_post_custom_values('picture');
  		foreach ( $picturevalues as $key => $value ) {
   		?>
         	<div class="picture">
				<img src="<?php echo "$value"; ?>
            	" width="196" height="132" alt="flowers" />
          	</div>
		<?php }?>
<?php endwhile; endif; ?>

This works fine UNLESS the page has more than one post (like the news page).

That is: when the page is a PAGE, (with no blog or other posts on it) it’s fine. But having other posts (even without the custom field called “picture”) it breaks.

It works fine here:
https://www.coastalmainehospice.org/wordpress/welcome-to-coastal-family-hospice-volunteers/

but gives me an error here:
https://www.coastalmainehospice.org/wordpress/news-items/

Also, it’s less than perfect when the custom field is left empty.

Thanks in advance,
Jim