It depends on when the pictures were taken and where they were posted. If the pictures were taken during work or at official work functions, and they were posted to the company's website or used for official company advertising or other promotional purposes, then yes, employers can legally do that. Outside of those situations, the issue becomes more grey and it depends more and more on the specific facts of each situation.
Anyone using another's “name, voice, signature, photograph or likeness” for commercial purposes without consent is liable for the injured party's actual damages or $750, whichever is greater. [See Civ.C. § 3344; CACI 1804]
More facts are needed to properly assess your case.