In the days of Noah, God was furious, he sent a flood to destroy the inhabitants of the earth. And he did. He destroyed every breathing person, animal, thing that wasn't on Noah's Ark. That is an intensely dreadful thing to think about.
And after all of this, God gave us, gave them the rainbow as a promise, a covenant that he wouldn't do anything like that again.
The Gospel, the good news of Jesus on the cross, our Savior coming to us in the flesh... He is our new rainbow covenant. He is our new image, our new thing to look to to remember the love and grace of our God.