In your code excerpt you have a function to add theme support for post thumbnails but you don’t include a line where you actually run it. Can you confirm that you are also executing wpb_theme_setup() as well as defining it? If it’s the only thing in the function, you could just call add_theme_support( 'post-thumbnails' ); on its own (that is, remove the wrapping function).
The closing ?> is not compulsory in PHP