I ran word counts on the Bitcoin whitepaper and Mastering Bitcoin.
In the Bitcoin whitepaper, the most-used word is transaction(s) — 69 mentions.
In Mastering Bitcoin, the most-used word is also transaction(s) — 1,534 mentions.
The whitepaper has a dedicated section on transactions, and Mastering Bitcoin has an entire chapter on transactions.
Bitcoin’s core purpose is to record and verify transactions. Without transactions, the network doesn’t really have a use case.
You won’t find chapters dedicated to storing files or images because that isn’t what Bitcoin is designed for.