Have a look at http://imgseek.python-hosting.com

Download this and give it a try - I did and the results were impressive on my photo collection.

Your correct; By drawing a cat, your not searching for a cat, simply images which are similar to your drawing. One could build upon this with the addition of meta data & reference images. Which would add some context to the searches and could actually be very powerful.

There are other ways to look at, being shape/path recognition etc. Using the discussed technique could also have a "Find Similar Images" feature- which for say an online photo gallery would be really neat.