ahh you forgot to mention that jpg is best for photos, as in not for artwork thats more linear in colours!!!! :P

personaly i use png a lot, you can chose the colour depth and with certain types of images it can be smaller than a jpeg, though tends to be on average a tad bigger.

Png is s'posed to be the new open standard to beat all other standards, well will wait for ie7 to come out so that we have a bit better alpha transparency support