Reverse image searching is where you upload an image and search based off that image.
So, why would you want to do that?
First, you can use reverse searching to try and find the original source. Once you find the original source, you'll have a better idea if you can legally use it or not. Another advantage is that you can see if you have an original or an altered version of the image (e.g. has yours been cropped?). Also, it'll give you an idea how often the image is used. If you're trying to create an unique presentation for class, you may not want to use an image that has appeared all over the Internet. Finally, you may discovered similar styled images that actually work better for your project.
TinEye was the first image search engine that allowed you to search by image. While TinEye does not pull similar results in the way that Google does, it does give you a much better idea of how a particular image is being used on the Internet and how often. Not only that, you have the option to compare your original image to the images it finds. This makes it easier to find any alternative versions or even to find if others are editing the images you put on the web.
TinEye allows you to easily switch between matched images and your original.
In addition to searching the TinEye homepage, you can download the TinEye extension for Mozilla and search any image you find with the right click of your mouse.
To do a reverse image search in Google Image, click on the small camera icon in the search bar. This will give the option to upload an image or enter an URL of an image already on the web.
After you enter an image, Google will either tell you what the image is or show you similar looking images. This is a good way to browse images in a particular style. For example:
For more information on using Google Image, visit the Google Image Page of this guide.