Tool for searching images

I've been looking for a tool that would search for picture's origins in the internet. If I have a picture on my computer and want to find out where is this picture from. What could I do, except asking ppl or trying to guess keywords in google? Is there a tool which could help me?

Before google, I guess, the same imgSeek people will finish their own project of global search: http://www.imgseek.net/net/index.html
They have an interesting idea - to make the searching distributed. This is good because of many benefits:
- Search speed and db space distribution
- Many picture packages are shared only through p2p networks, not the www
- Web crawlers are slow/expensive

The bad side of distributed search is that doubtly many people will download and use such a tool, unless it will be included as a part of some bittorrent client.

But anyway, the project seems active and promising.