pathawks/Category-Thumbnail — pull request #1 by CAYdenberg (2014-11-20 18:48:57+00:00)
Title: Added options for custom post types and sizing of the thumbnail/featured image

See size options in http://codex.wordpress.org/Function_Reference/get_the_post_thumbnail
For this purpose needs to be a string (obviously).
To avoid PHP warnings, made sure that if there is no $post[0], an empty string is returned.