A WebmasterWorld thread has some members who focus on watching Google Image results stating that Google Images is now better at figuring out who the original source is.
One SEO said:
I'm doing the happy jig at the moment, my competitor just had the snot beaten out of his/her image rankings. For years they would have a set of top images in search for every query but the images were hotlinked from their sources (primarily classifieds sites, galleries etc) and not really hosted by the competitor.
While doing a routine check of my image rankings for particular keywords I noticed that the top 4-5 images for one search had remained the same but the source had changed. These used to be my competitors rankings, but not anymore, now the source was ranked.
Others also confirmed the change.
I am not sure how many people noticed this or not but I certainly hope it is true.
It is terribly challenging for Google to find the true original source of an image or even content. If they did, at least even 75% of the time, I would be amazed and in awe.
Forum discussion at WebmasterWorld.
Update & Confirmed: Google has confirmed this saying:
"We've recently launched an update to the algorithm that looks at the relevance and quality of both the webpage and the image to surface more relevant results in Google Images. Improving the quality of results in Google Images is very important to us, and we're always working hard to improve our algorithms.