It's a bit of a roundabout journey:
1. "Eikon" became the Latin word "icona".
2. "Icona" then became the English word "icon".
3. Finally, "icon" was modified to "iconic" to create an adjective meaning "relating to or resembling an icon".
So, while the word "iconic" is now used to describe anything that is widely recognized and significant, it ultimately traces back to the Greek word for "image".