Actually Karl, we recently made a change to fix a bug in referer URLs that were not correctly showing some referers.
Those image referers you are seeing are users who did an Image search on google, like what they saw and clicked on the "see image in it's original page"... So those are legit visitors... they're just getting to your blog in an unusual way.
The bug we fixed was incorrectly placing these referers into the "internal" category. Now we correctly show you where those readers really came from.
Oh, and now I know why that crazy 4th of July post is so popular... apparently a lot of people are searching for images if Shetland Ponies. Go figure.