September 04, 2014
Often, the WordPress Plugin Multiple Post Thumbnails is used within the page.php and single.php templates, and few conflicts arise. If you’re calling your thumbnails within header.php, footer.php or functions.php, the following code will help sort out issues with images not displaying via the simpler methods.
if (class_exists('MultiPostThumbnails') && MultiPostThumbnails::has_post_thumbnail('post', 'secondary-image',$post->ID)) : MultiPostThumbnails::the_post_thumbnail('post', 'secondary-image', $post->ID, 'secondary-image');
endif;
if (class_exists('MultiPostThumbnails') && MultiPostThumbnails::has_post_thumbnail('page', 'secondary-image',$post->ID)) : MultiPostThumbnails::the_post_thumbnail('page', 'secondary-image', $post->ID, 'secondary-image');
endif;
if (class_exists('MultiPostThumbnails') && MultiPostThumbnails::has_post_thumbnail('custom_post_type', 'secondary-image',$post->ID)) : MultiPostThumbnails::the_post_thumbnail('custom_post_type', 'secondary-image', $post->ID, 'secondary-image');
endif;
Sourced here in the WordPress support forums.
