I can't change my thumbnail

I published a post and then later changed the main (and only) image in the post, but the thumbnail on the home page does not change even though I specified which thumbnail to use in the images option in post edit screen. ARGH! Yes, I’ve cleared my browser cache. Still no change. How do I delete the old thumbnail?
Thanks
the post: http://www.growingababymomma.com/how-to-be-a-good-parent/
How it looks on the front page: http://www.growingababymomma.com/