I am having a similar issue. See: Featured Image Size and Cropping Elimination
I have read through post upon post about this, both issues and codex's. Still have yet to find a concrete solution.
I have added the set_post_thumbnail_size and configured the Media settings.
<?php echo get_the_post_thumbnail( $post_id, $size, $attr ); ?>
Where does this go? single.php? I see nothing much that resembles it in the code...
[Code moderated as per the Forum Rules. Please use the pastebin]
Isn't this featured image a basic inherent functionality of WordPress? I even read somewhere that the medium thumb was the default. I am not a PHP code-monster and really need some help with this!