If you're breastfeeding, you can also express some of the milk with a pump and use the milk to wash the acne. Milk has all sorts of wonderful antibodies that help the acne heal.
How old is your baby? If just recently born and the pimples are slightly whitish in appearance they may not be acne, they may be "Erythema toxicum"- a harmless rash that I think is caused by bacteria from being born. It clears up on its own.
Most of the time the best solution for rashes is to make sure the baby isn't allergic to anything in his/her diet (try a different formula or if you're breastfeeding try the "Elimination diet") and avoid using baby bath products. Just wash with simple warm water and don't use baby washes (Johnson's always gave my son a pimply rash) or soaps. I was able to use plain Bronner's unscented castile soap after my son was about 4 months old. And even now I only use it when he's starting to smell pungent. Baby skin is delicate and can't really deal with too much washing.
Diaper rash needs to be taken care of with product since it's irritation caused by chemicals in the urine and poop. Use a thick creamy diaper rash cream or something like Butt Paste (if the rash is really bad) but most other rashes you just have to find the cause and then leave it alone to dry out.
Oh- you can also try switching laundry detergent (try Dreft or other hypoallergic detergents. My son was horribly sensitive to "All" and broke out everywhere even after we re-washed his clothes. We had to wash them twice to get all the residue out.)
Good luck.