Apple caps free iCloud at 5 GB except for Shared Photo Albums which count as 0 bytes against your quota. You don't even have to invite anyone to the album.
• 200 Shared Albums per Apple ID
• 5,000 photos per album = 1,000,000 slots
• Max resolution: 2048×2048 px
icloud-s3 packs 3 raw bytes into every 24-bit RGB pixel (R, G, B) of a lossless PNG (~12.18 MB per 2048×2048 frame) with a 64px HUD header + CRC32 & SHA-256 verification burned right into the image.