In trying to make this game, I did lots of looking into Pictish Symbol Stones.

Here are some links to things I found particularly interesting.
(I watched most of the videos, but not the full playlists or books.)

Some Videos

• Pictish Gods - Kelly Kilpatrick
• Pictish Writing & Pictish Symbols: Decoding the Newton Stone - Kelly Kilpatrick
  
• Towards Establishing the Rule Book of Pictish Symbol Usage - Hugh Levey
  
• The Development of the Pictish Symbol System - Gordon Noble
• Discovering the Picts - Gordon Noble
  
• Discovering the Northern Picts - Gordon Noble
• The Drosten Stone, St Vigeans: a cultural hybrid - Jane Geddes
• The Picts - John Borland (maybe a good starting point)
• Warriors and War of Pictish Sculpture - John Borland

Kelly Kilpatrick has all her videos available on her website. They are all really interesting, I'd recommend watching them!

Full Playlists

• The Picts 3 Video Documentary Playlist (Probably a good place to get started too, especially if you're not a fan of presentations)
• Full Playlist on the recently discovered (in 2019) Conan Stone.

Books

• A King In the North - Gordon Noble
• Picts: Scourge of Rome, Rulers of the North - Gordon Noble

Organisations

• The Pictish Arts Society (PAS)
• Scottish Society for Northern Studies (SSNS)
• North of Scotland Archaeological Society (NOSAS)

Databases

• Canmore - Database of historical sites + finds in Scotland. (I've linked the search for 'Pictish') . The survey drawings they have of the stones by John Borland & Ian G Scott are really amazing. It takes a bit of digging but you can check the image tab for a particular site to find them.