137 songs from the past albums -> the skull
The future -> the robot
Must be the band signaling that they're moving on from their past and speeding (distance over time) into the future.
I think they are emphasizing that their careers, but mainly life in general, went by quickly. Before you know it, your'e dead, never knowing what the future
holds In Ferris Bueller's Day off, he says "Life moves pretty fast. If you don’t stop and look around once in awhile, you could miss it.", and in that movie's 137th section of the script, Ferris is on the float having the time of his life. He's singing "Twist and Shout", a song written by a band that is probably liked by and an influence on the members of Dream Theater.
https://www.idiotsavant.com/bueller/script.htmSo the symbolism on this album cover, obviously, is our future overlords looking at our remains, relishing in the fact that unlike us humans, their time does not move by as quickly. They don't need to stop around and look around once in a while, because they are all knowing. They look into our empty eye sockets, constantly curious of what that feeling must have felt like, wondering whether or not they are truly living.