It seems that anything can be “iconic”. I’ve collected all of the following in just a few weeks:
The turtle is an iconic species.
The beauty of a traditional radio telescope is iconic.
Al-Qaeda targets iconic buildings.
The Roundhouse, in London, is iconic, as are the ruins of Greyfriars at Dunwich (both obviously to be targeted by Al-Qaeda!)
Terence Conran is Britain’s most iconic designer.
The gull-wing doors of the Mercedes 300 are iconic.
Paul Henreid lighting two cigarettes in “Now Voyager” is an iconic image.
The OED has no earlier quotations than 1976 for what one has to assume is the meaning in all these cases:
“Designating a person or thing regarded as representative of a culture or movement; important or influential in a particular (cultural) context”.
How quickly meanings can spread.