I responded you yesterday here, and on your email, but looks like something went wrong and answer was not posted here.
I’ve updated theme, and you should first re-download it. I added some extra styles for WordPress gallery.
I also sent you 3 files on email which you need to replace in this new theme (that will enable cropping).
Once you do that, regenerate thumbnails with wp regenerate thumbnails plugin.
For the posts where you don’t want featured image, but you want to show images – use gallery. Put images inside the gallery, preceding by a read more tag, and it will work as you imagined (you’ll also get more consistent look).