featured image

I cant figure out how to edit featured theme. I really just want to crop out a part of my primary post picture but I cant figure it out. I think I am pushing the crop button but it never seems to save the cropped version as the featured theme. I am trying to do the thumbnail the same way…just a cropped version of the mail pic. Is this doable? How? I am using twenty fourteen and my site is comiccon-costumes.com if it matters.
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