The images, which show multiple women posing provocatively in front of a fire truck and wearing Atlanta Fire Rescue gear, appeared on the website Backpage Georgia, Channel 2 Action News reports.
Backpage, an online classifieds portal, has been accused of facilitating sex trafficking.
"The Atlanta Fire Rescue Department will launch an internal investigation into the photos and will take decisive action based on the outcome of the investigation." Atlanta Mayor Kasim Reed said in a statement.
One of the risqué photos shows a woman wearing a fire jacket on top of a fire engine, while another shows a woman in a firefighter's locker.
One ad reads: "THE REAL DEAL. Look know (sic) further! Satisfaction Guaranteed!"
In another set of photos, a woman spreads her legs around a siren on the top of a fire truck. "Available Now. Come experience this hottie. CALL NOW!".
The head of the Atlanta Professional Firefighters union said employees should not be punished for the incident, the Atlanta Journal Constitution reported.