feat: Artwork for SteamGridDB and Lutris (#100)

Adds colorful artwork primarily intended for use in game libraries.
Assets have been submitted to their respective target sites:

- https://lutris.net/games/keypunch
- https://www.steamgriddb.com/game/5476734 (not accessible until it's
accepted)

Based on the existing Discord RPC picture (now renamed to square.png).

Author: bragefuglseth