Fix display thumbnails function

This function stopped displaying thumbnails somewhere along the way. `display_post_thumbnail_src()` was creating an array of urls to thumbnails in the post, and `display_post_thumbnail()` had a mysterious `$src` param with a value that could never be equivalent to the output of `display_post_thumbnail_src()`. So it displayed nothing. This fix gives us one image url or the `DEFAULT_PHOTO` constant returned from `display_post_thumbnail_src()`, and `display_post_thumbnail()` checks if `display_post_thumbnail_src()` has a value and displays it along with the right class.