“It’s like deja vu all over again.”
Sometimes you have saved an image that you want to use in a presentation, or on your blog, and you have no idea where it came from. One easy way to find a source for an image (no guarantee that it’s the right one) is Google’s Image Search. It’s not only good for typing search terms and getting images that match these terms (or not), you can drag in images as well and search for places on the web, where the image appears.
Here’s an example (yup, I know where that image is from, actually a very good xkcd comic, but just to show the principle):
The image is uploaded (who knows what Google or the NSA does with it, so use it at your discretion) and Google tries to find the same image online — or very similar ones. For photos you frequently get images in the same style/color scheme, but it is surprisingly accurate and fast. You can also search for other, e.g., better quality versions of the image.
BTW, it’s not only useful for finding the source of images, it is also surprising effective in online dating. Many people use photos they have uploaded somewhere else for their online dating profiles, and this way you can quickly get some more information about the person (things like name, workplace, etc.). Yup, it’s scary and might be seen as cyberstalking — on the other hand, if you do not misuse it, then it gives you some more confidence and trust that the person is actually who s/he is claiming to be. After all, there are a lot of fakes online. But if you do not want to be found, better use photos in your semi-anonymous profiles you use nowhere else on the Internet.