Probably, it means the mental pictures (images) we have of certain things (like, in this case, firemen) are no longer appropriate because the world has changed. Many of our mental pictures are based on impressions gained when we were young. The world is now changing so rapidly that many of these early-formed images no longer fit the reality of today.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.