I had a similar problem and found this small workaround:
Using Photoshop or a similar image manipulation software, crop a desired section of your image to 600px X 200px. You'll then upload two files into your post's gallery. The original, full image. And the cropped 600px X 200px edit. Insert the full-sized one into your post as you normally would. But make the cropped one a featured image, without inserting it into the post.
It's a little more work, but this way you can control how your post feed looks, and when people click into it, they'll only see your full-sized image.
This would fix things like this in your posts: http://erictien.wordpress.com/2012/09/25/first-impressions/