Even if visible in plain sight, the feature to search by images in Google is less known and it can be incredibly useful and a good alternative to some other less accessible services. Let's have a look at using it.
Start at the Google image search at http://images.google.com/. Did you know you can drag images from your desktop or URL from other windows into its search box?
Or if you are less accurate with dragging, the search box has a photo icon which you can use to copy/paste URLs or upload images? Good to know on both accounts.
When researching the feature, my first idea was to try a picture of the supermoon I recently shoot in Vienna. The result is accurate, Google identified it as a full moon (technically, a supermoon is a full moon), offered good links, including a Wikipedia article and some similar images (is the moon, it looks alike in all photos).
Then I wanted something different, so tried an image with the old Mozilla Firebird logo I had lying around from another recent article. Yet another success, logo recognized correctly as a logo, as Mozilla's, good links, etc.
Of course is not all roses, when trying with another picture which is 1.) harder to identify and 2.) not available on the Internet, the result can be funny (and totally off - I don't think I am a horse)
But still, for pictures not available online, it can be quite good and still useful, here it identified pretty accurately two person in black dresses (or at least two black things) doing something together.
Fun, fun, but I said it can be useful, let's do something useful, like searching for
stolen inappropriately used/licensed images. After the supermoon above, my second try was a photo with cherries which a while ago I used as my desktop wallpaper and also posted in a few places (the cherries season is close!). Yet another success, Google identified it not only as cherry, but as sour cherry, which it actually is, probably based on the content of the pages using it. A good description taken from wiki, a relevant first link and a bunch of related pictures and then a link to my photography blog hosting the image. Not bad at all.
Now let's look further: the image can be found in a few Wiki pages, which is normal, I uploaded it to Commons myself. Then the fun begins (the image originate from my blog with a CC-BY-SA license or from Wikipedia, also with a CC-BY-SA license).
There are pages where the image is hotlinked directly from my server (so I could have detected its use from Apache logs or even could have blocked external referers, but that would have been evil). Of course, no links back, no credits, no nothing. The fun part: I can see with my bare eye another cherry picture on that page which is taken from my blog.
And there are pages hosting their own copy of the image, some with edits (like adding some text on top) and some at large resolution (those are taken from Wikipedia instead of my blog).
I won't police those sites, the picture was up for share (albeit under a different license, not at Public Domain) and this article was not intended to whine about copyright offenders, but to show the use of an image search tool. Have fun with your own searches!