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I'm trying to upgrade to a new verions of WordPress and get an error about `MultiPostThumbnails`

This is caused by using the example in previous readmes that didn't do a check for the MultiPostThumbnails class existing first. This has been corrected in the Installation section.

How do I register the same thumbnail for multiple post types?

You can loop through an array of the post types:

if (class_exists('MultiPostThumbnails')) {
    $types = array('post', 'page', 'my_post_type');
    foreach($types as $type) {
        $thumb = new MultiPostThumbnails(array(
            'label' => 'Secondary Image',
            'id' => 'secondary-image',
            'post_type' => $type
            )
        );
    }
}

How do I use a custom thumbnail size in my theme?

After you have registered a new post thumbnail, register a new image size for it. e.g if your post thumbnail id is secondary-image and it is for a post, it probably makes sense to use something like:

add_image_size('post-secondary-image-thumbnail', 250, 150);

This will register a new image size of 250x150 px. Then, when you display the thumbnail in your theme, update the call to MultiPostThumbnails::the_post_thumbnail() to pass in the image size:

MultiPostThumbnails::the_post_thumbnail('post', 'secondary-image', NULL,  'post-secondary-image-thumbnail');

You can register multiple image sizes for a given thumbnail if desired.