By default, ‘medium’ should return a scaled down and un-cropped image. And I don’t know why “full” wouldn’t work for you either.
Give this a try. Set your Return Value to Image URL and call it like this…
<?php the_field('ceo_background'); ?>
This will return the URL to the un-cropped, unscaled image. If you see that it’s scalled down or cropped, then you know that either the JS or CSS is modifying the returned image.
