Iíve just recently painted a few rocks with acrylic, itís fun! DecoArt makes an outdoor acrylic paint that should work especially well for rock painting:
If your painted rocks will be outside, then they should be sealed with a clear, gloss, waterproof sealer on every side to prevent moisture from soaking into them and bubbling your paint. I got spar urethane to seal my rocks, but I haven't tried it out yet and don't know if it's the best outdoor sealer for them.
Check out youtube for lots of rock painting videos. My favorites are the rock mandalas.