When you save the image from the webcam page it's already a .jpg so there's no need to convert anything. Now, when you resize, if you use a program like
THIS ONE it won't change the format so when you start with the saved jpg you'll end up with a jpg.
Sorry I didn't post that link sooner... I've been busy at it totally slipped my mind. The program is really simple and easy to use (and it works in batches too, which is very handy). Check it out, you'll see how easy it is.