The widget above uses TinEye to create its effects, but you can use it to make images search for images. Here’s what TinEye says about itself:
TinEye is the first image search engine on the web to use image identification technology rather than keywords, metadata or watermarks. TinEye regularly crawls the web for new images, and we also accept contributions of complete online image collections. To date, there are 2,221,049,210 images in our image database.