Drag and drop an image from the web or your computer into the search box or
On images.google.com, click the camera icon, then select “Upload an image.” Select the image you want to use to start your search or
Found an image on the web you’re curious about? Right-click the image to copy the URL. On images.google.com, click the camera icon, and “Paste image URL” or
How Google Search By Image Works?
“Google uses computer vision techniques to match your image to other images in the Google Images index and additional image collections. From those matches, we try to generate an accurate “best guess” text description of your image, as well as find other images that have the same content as your search image. Your search results page can show results for that text description as well as related images.”
Note : Extension will be fully functional at 6 PM, PST on June 15, 2011.
* Firefox 5 not supports. [Via]