Petiteto

I found the problem:

You need to replace this line in the theme functions.php:
add_theme_support( 'post-thumbnails', array( 'post', 'slides', 'wppetfinder_pet' ) );

To:
add_theme_support( 'post-thumbnails', array( 'post', 'slides', 'wppetfinder_pet', 'pet' ) );