The 15 heroes in the portrait, left to right, are: Star-Spangled Kid, Wildcat, the Atom, Sandman, the Spectre, the Flash (Jay Garrick), Hawkman, Hawkgirl, Dr. Fate, Green Lantern (Alan Scott), Hourman, Dr. Mid-Nite (with Hootie the owl), the original Black Canary, and Mr. Terrific.
John Jones (aka Martian Manhunter) remarks to Oliver that they did not have dessert on Mars, but since being on Earth, he has developed a fondness for cookies. This is another nod to the fact that in the comics, the Martian Manhunter is quite partial to Oreo cookies.
The antagonism between Green Arrow and Hawkman mirrors the characters' relationship in the 70's and 80's era Justice League comic book, where Green Arrow's liberal politics were often in conflict with the conservative Hawkman.
Dr. Fate calls the future where Clark becomes Superman as a "Silver Age of Heroism". This is in reference to the Silver Age of Comic Books, a resurgence in the popularity of superhero comics in the post-Korean War era which ran from the mid-1950s to the 1970s.
When Clark is exploring the museum he uncovers a round table that the Justice Society of America once used. A nice reference to King Arthur and his Knights of the Round Table. A round table, because all who sit at it are equal.