it does require signup but it's pretty quick.
www.tineye.com
"Every day TinEye's spiders crawl the web for additional images. Using sophisticated pattern recognition algorithms, TinEye creates a unique and compact digital signature or 'fingerprint' for each one and adds it to the index.
When you want to find out where an image is being used on the web, you submit it to TinEye. The attributes of the image are analyzed instantly, and its fingerprint is compared to the fingerprint of every single image in the TinEye search index. The result? A detailed list of any websites using that image, worldwide. "
www.tineye.com
"Every day TinEye's spiders crawl the web for additional images. Using sophisticated pattern recognition algorithms, TinEye creates a unique and compact digital signature or 'fingerprint' for each one and adds it to the index.
When you want to find out where an image is being used on the web, you submit it to TinEye. The attributes of the image are analyzed instantly, and its fingerprint is compared to the fingerprint of every single image in the TinEye search index. The result? A detailed list of any websites using that image, worldwide. "
