Absolutely helpful, practically one-of-a-kind Rated 5 out of 5 stars
Haven't seen anything like this in … ever. It is very thorough, able to find images that have been edited, resized, cropped, or vary in color. It even finds "meme" versions of images, like (de)motivators, macros, and inserts in comics.
Have a tiny thumbnail? You can probably find a wallpaper version that goes with it, if it's out there. Produce your own artwork or photos? You can find those [negligent souls] who are using (or selling!) your work without permission.
Only bad thing is that their catalog doesn't cover every single image ever created. ("Oh boo hoo, it didn't find my one photo I just took so I'm rating it 1 star…")