I have a twitter feed and I can find the instagram url in a tweet.
{ indices: [ 90, 110 ], url: \"http://t.co/kF9EXifn\", expan
You can get just the image by appending /media/ to the URL. Using your example: http://instagr.am/p/QC8hWKL_4K/media/.
/media/
You can even specify a size,
One of t (thumbnail), m (medium), l (large). Defaults to m.
So for a thumbnail: http://instagr.am/p/QC8hWKL_4K/media/?size=t
Documentation here.