Answer:
Historic/traditional.
Step-by-step explanation:
"Millions of users attempted to log into twitter only to be greeted by the services iconic "fail whale," the image of a cartoon whale being hoisted into the air by a flock of birds, signaling a site outage."
Based on the passage, it is evident that whenever the twitter site crashed, the image came up which has become famous and well known by now. And with that, the already famous image of the "fail whale" greeted anyone trying to access it. In order to change the word "iconic" into another word but still retain the meaning and status of the replaced word, the synonym "historic or traditional" will be best. "Iconic" suggests something that's super famous and capable of exuding awe and grandeur, which "historical/ traditional" can also do.
So, the word "iconic" can be replaced by "historical/traditional".