if (class_exists('MultiPostThumbnails')) {
            new MultiPostThumbnails(array(
            'label' => 'Secondary Image',
            'id' => 'second-image',
            'post_type' => 'project'
            ));

            new MultiPostThumbnails(array(
            'label' => 'Tertiary Image',
            'id' => 'third-image',
            'post_type' => 'project'
            ));
}
add_image_size('project-second-image-thumbnail', 100, 100);
add_image_size('project-third-image-thumbnail', 100, 100);