when using your theme on mobile devices (if there is not enough resolution), pictures in the post are resized only horizontally (keeping their vertical size unchanged) which looks kinda strange as they have improper aspect ratio.
The featured image must be bigger than the minimum resolution of thumbnail for proper cropping to take place. If the images are big already and still thumbnails are of different sizes then you have run this plugin once https://wordpress.org/plugins/regenerate-thumbnails/
There is one more option in case it suits your site.
line 54 of functions.php is
add_image_size('excerpt-thumbnail', 200, 140, true); // Sets Index Page Thumbnails
You can try to stop cropping from center by setting true to false.
add_image_size('excerpt-thumbnail', 200, 140, false); // Sets Index Page Thumbnails
Thank you for your quick reply. I forgot to add that I plan to use your template to upgrade my old blog with 1000+ posts with images sourced externally via link. They are not featured nor embedded into WP database.
I did try to build my theme in Genesis with Dynamic Web Site builder before I found about Iconic One Pro and scaling of theme was OK.