Fire bosses have vowed to find out more about the incident and the people involved after the snap - taken in Manchester - was reportedly posted on a swingers' website.
It was one of a number of X-rated images posted a man who claimed to be a firefighter and was seeking "fellow swingers", it was claimed.
The snap has sparked outrage as Greater Manchester Fire and Rescue said staff will be "embarrassed at best" after the picture emerged online.
Fire chiefs confirmed they are investigating the "inappropriate" image which was published in the Sun.
It shows a woman topless in the cab of a fire engine as someone grabs one of her breasts.
Fire equipment can be seen inside the engine, which bears the service's website address on the door.
The snap was posted on a profile by someone who claims to be "6ft 5ins Callum, 27” and "genuine fire crew".
The man wrote on the profile that he was seeking women aged 18 to 60 for "adult parties, blindfolds, dogging and group sex".
A Greater Manchester Fire and Rescue spokesman said: "I cannot believe we're still seeing highly inappropriate images like this in the fire and rescue service in 2017. It's not who we are.
"Firefighters in GMFRS are hard-working and proud to serve their communities. They are professional and diligent. Crew will be embarrassed by this at best.
"Most people will be furious at the suggestion this is acceptable or a reflection of how we behave. It isn't.
"We'll do everything we can to find out about how this picture was made and who was involved. There is no place for this in our organisation."