Try this:
open Appearance -> Editor -> functions.php
on line 51 you will see
add_image_size('excerpt-thumbnail', 200, 140, true); // Sets Index Page Thumbnails
Replace it with
add_image_size('excerpt-thumbnail', 200, 140, false); // Sets Index Page Thumbnails
We just changed true to false.
true: images will be cropped : this is the default
false: images will be scaled : can cause uneven sizes if your parent images are of different sizes.