Welcome to therichpost.com. In this post, I will tell you, How to Add custom attribute to wordpress the_post_thumbnail? Like I always say WordPress is the best cms.
Here is the working and tested code and you need to add this with your the_post_thumbnail:
the_post_thumbnail( 'thumbnail', array('itemprop'=>'image') );
There are so many tricks in wordpress and i will let you know all. Please do comment if you any query related to this post. Thank you. Therichpost.com