How to Add custom attribute to image using the_post_thumbnail
Sometimes you will need to add a custom attribute to an image. This is very easy to do using
the_post_thumbnail() function. The following snippet will add the custom attribute of
'itemprop="image" to your images but you can change that to nearly anything including
class, alt, title etc.
WORDPRESS SNIPPET : PHP
the_post_thumbnail( 'thumbnail', array('itemprop'=>'image') );
( WordPress codex functions, hooks, in this snippet. )