Ok ok. I have now seen the episode in HD, and inspected frame by frame.
Things that are on the wall:
- The Ruins of Polis
- King Roan
- Luna
- Murphy from 3x16
- Grounders in cages
- The Waterfall (that Finn & Clarke found I think)
- Lexa in warpaint
- A room (under the waterfall) that looks like the MW infirmary but I'm not sure
- Bellamy from the control room in 2x16
- Abby
- Something that kind of looks like the shocklashing of Abby with Kane watching?
- The top of what I assume is Lincoln's head
- Another Murphy
- Madi in the collar
- A grounder hanging in MW
- Another Madi
- Clarke pulling the lever in 1x13
- The Rover
- The Vulture from 5x01 (I think)
- Anya in 2x04
- The Arkadia sign
- Lincoln being tortured
- Clarke after the Jonty prank
- Skeletons (1x13?)
- Bellamy (1x02)
- The two-headed deer
- Maya in Jasper's arms
- Chipped!Raven bound to the bed
- Lexa in bed
I'm sure someone already made a complete list but it was fun to test my own knowledge.
There's a couple that were either not completely in shot or so vague/blurred that I couldn't make out.
I'm also not sure what this one is:
