Okay, someone sent me a Japanese painting from the 1600s and asked if I knew the artist. I didn’t recognize it, so I searched.
I used Image Search at Google and Baidu. You can drop an image (photo, painting, etc.) (or use your phone to take a photo of a painting) in these.
Try it. Find a very obscure painting (say, a Dutch sea painting of the early 1700s). If the image is on the web, these tools will find it.
There is also Tineye.com.
Another useful Google image tool is Advanced Image Search. Need an image for your Powerpoint or website? This tool lets you search for images without usage-restrictions.