Gallery Thumbnails

I have a gallery on a page with about 9 or 12 photo images. Some of the image thumbnails slice off the heads of some of the animals. Via the media editor you have a full side image to one side and the smaller thumbnail to the right. You can drag the cropping square to your desired size on the full size image and it says you can check the box to apply only to the thumbnail, but I can’t seem to get it to work????

Any ideas!

Thanks so much in advance!