I've updated functions.php to make the image sizes portrait instead of square for medium thumbnails:
$k_option['custom']['imgSize']['M'] = array('width'=>180, 'height'=>250); // big preview pic for full sized posts
I updated the CSS to make the rounded border fit my desired image size 180px wide by 250px height.
Next I installed the plugin to regenerate the thumbnails and ran that for one of my images but it didn't change. I also uploaded a new image which is 180px by 250px. I selected 'full size' and 'set as featured image' in the set featured dialogue box but is still appears square?
You can see here: http://www.nicolamorgan.com/author/uncategorized/test-post-image-size/