Vector Magic is the best website I've seen so far for graphic vector conversion. The image loss after conversion is relatively small, so there's no need to install large software like Illustrator just to convert a photo. The previous beta version allowed unlimited conversions, but recently the official version has been released, and it seems that only a few photos can be converted for free. The number of conversions is calculated based on points. You can earn 2 points each time you log in. If you run out of points but still need to convert, you need to register and pay. However, you can register multiple accounts to use it for free :P