Upon activating Theme and migrating from another theme, previously added featured images might take strange aspect-ratios, be unnecessary big or just look out of place.
This is because image thumbnails have not been resized/cropped.

If you are loading this theme onto an existing website, we’d recommend running the “Regenerate Thumbnails” plugin which will go through all of your images and resize according to this themes sizes.

You can find the plugin by typing in “regenerate thumbnails” into the plugin search (Plugins > Add New) and installing from there. Alternatively you can download from: regenerate-thumbnails plugin