Soon there will be new UI of Steam and it would be good to create new images so Chicken Invaders presents well there.
Also there’s now GOG 2.0 beta and both use vertical images (like old game boxes so that’s awesome). I linked my Steam account which imported my games and here’s how Chicken Invaders presents on “All games” tab:
Good thing to observe here is that CI5 DLCs (I only have CI5 with every DLC so maybe previous parts also) count as different game (probably due to different executable), but still have the main game art.
So yeah. It’s not the most important thing right now (not like Apple’s app policies where you had to stop all work and do it), but keep it in mind.
EDIT: I forgot about official guide: https://steamcommunity.com/groups/steamworks/announcements/detail/1597002662762032240