Cornerstone v1.0.3

Featured images were being force-cropped to 1200×600 regardless of original aspect ratio, causing images with non-standard dimensions to lose significant content.

The theme's functions.php used hard cropping for featured images:
phpadd_image_size('featured-image', 1200, 600, true);
This caused issues with:

1200×1245 screenshots: Severely cropped, losing ~645px of height
Portrait images: Lost top/bottom content
Square images: Forced into landscape format

Images with 1200×833 (landscape) worked because they were closer to the 2:1 crop ratio.
Solution
Changed to soft crop with flexible height in functions.php:
phpadd_image_size('featured-image', 1200, 9999, false);
This maintains aspect ratios whilst limiting width to 1200px.
For Users Who Installed v1.0.0
If you installed the theme before this fix:

Update functions.php with the corrected code
Install Regenerate Thumbnails plugin
Run the regeneration process
Existing images will display at their correct aspect ratios