What study showed that humans can successfully detect objects within images once they are able to recognize those objects?
Using Google's search by image feature, I found that the image is usually entitled Richard Gregory dalmatian.

From there, I found this page, which cites the image as coming from Gregory's 1970 book The Intelligent Eye. There's also, incidentally, a good list of Gregory's publications on this personal web page, which is probably worth looking at.

From a theoretical perspective, the image is a demonstration of the role of top-down expectations in perception - we see not what's there, but a combination of the input to our retinas and what our prior knowledge tells us should be there. This is a case of Bayesian perception, and can be read up on in Kersten et al (2004), amongst many other places.