The image is an advertisement with the big green eyes of Dr. T. J. Eckleburg. The image is especifically the Green Eyes. Nick associates this image to Wilson's garage because he sees it at the front of the garage for the first time. Nick and Wilson both associate the eyes with the idea that God is watching (and judging) their actions. It is not by accident that the car crash that kills Mirtle happens there, since the idea is that the situation brings out the true self of every character