If you haven't already seen Google's search by image function you'll be amazed at what it can do. My only beef with Google is that over time they eliminate services many people (myself included) have come to depend on.
Tin-Eye's reverse image search engine was launched in 2008. I believe they were the first company to demonstrate a working system that could be used to search for duplicate or close-match images. It does not search the web per se. Tin-Eye's technology searched a database of images the company crawled using web search engine technology. They no longer provide the free demo search they once offered.
Two or three years ago Google launched reverse image search via its Image Search engine. Many people who regularly use Google web search don't realize that Google also provides several other specialized and free search engines. Image Search is one such engine.
The small camera icon located inside the Image Search box, at right, is the button that activates the reverse search function. Click the camera icon and you can either paste a web-based image address (URL) or upload an image file. With a second or two Google provides a selection of matches or close matches. There are even ways to refine the search.
For example, using Firefox I right-clicked on a news photo of a commercial jet aircraft and copied the link location. Then I pasted it into the "Past image URL" field and clicked search. Not surprisingly, Google displayed the same photo and article but below that there was a section for "Visually similar images." There's also a link for "Search tools." Click that and you get a few refinement capabilities including limiting the time-frame of the search results.
If you have the Android OS on one of your mobile devices there are apps that take advantage of the Google's reverse image search capability. One of those apps is Google Goggles. It has been around for a few years. It works sometimes and that's the best I can say for it. Google's Reverse Image search works much better.
Now for the fun (or scary) part. There will come a day, probably sooner rather than later, when you can reliably do real-time search by face image and find information about the person whose image you are capturing with your device. Google Glass may soon be able to reliably use instantaneous facial recognition technology to background a person. A company seeking funding claims it will do just that.
So, do we eagerly await the development or do we start writing our Congressperson to complain? That's a hard question to answer. Once the technology genie is out of the bottle it is impossible to put it back into the bottle and cork it up.
What I do see coming down the pike is a whole new industry of countermeasures, similar to current Electronic Warfare Self Protection systems.
Decades ago when anti-aircraft missiles were first guided by ground and missile based radar countermeasures were developed. During the 1980s I did some marketing work for the company that was developing the U.S. Navy's Airborne Self-Protection Jammer system. There's also a High-speed Anti-Radiation Missile that does the job by destroying the ground based tracking radar that guides some anti-aircraft missiles.
So, why not truly "disruptive" technologies that will defeat instantaneous facial recognition? I suppose if you employ them in the future you'd probably run the risk of being a suspicious person, someone who doesn't want his picture taken. I hope not.