My comp setup is as follows: - layer 1: image - layer 2: duplicated from layer one
I'm looking for an expression or any other method (Could be a plugin recommendation) so when I change the image of layer 1, it automatically updates layer 2!
It depends how you mean:
Option 1: If you put the image in Layer 1 inside a Comp, then any change to the image inside the Comp will change in both Layer 1 and Layer 2 (assuming you also put the Comp on Layer 2).
Option 2: You could put a load of different images inside a comp, a different one on each frame. Then in your master comp, place this comp, but assign Time Remap to the comp, to choose to hold on a specific frame. Then when you want to change the held frame, just keyframe to the different time in your first comp.
Option 3: If you really mean to change the image programmatically, you could use something like Templater Rig, where you can set a load of different file paths to images, and then assign them to different comps programmatically.