Those pimples may be acne or folliculitis. They both look like red bumps with occasional white centers.
If you ever had acne on the face and if you have pimples on the back for a prolonged time (months), then these may be acne.
Folliculitis is a staph infection of hair follicles, which may heal by itself. Mupirocin cream, which is an antibiotic, should cure it.