Records the books must support
IRS Publication 583 lists the documents that support gross receipts, purchases and expenses: deposit slips, invoices, receipts, canceled checks, account statements and Forms 1099-NEC among them. An outsourced bookkeeper should attach or link the supporting document as the transaction is coded, not at year end.
The same publication says the rules for paper records apply to electronic storage systems, and originals may be destroyed once the electronic system has been tested. Agree with the provider where those images live.

