How do cheques clear?
I know in the good old days, a human looked at the cheque, verified the numbers - dollars, account, bank - and then they approved the request by the bank that cashed the cheque to transfer the money. Request to pay was reconciled with payments, etc. etc.
I assume nowadays all the numbers are matched up electronically. The image is probably just a backup in case there is some question about the validity of the transaction? Or is it a matter of many banks still being tied to overnight batch processing? You cash at bank A, who processes transactions overnight and sends a consolidated list of requests to each other bank, including bank B where the cheque was drawn. The next night, B processes the request and notes it will send payment to A. The third night, A receives the payment batch list and processes that, including the payment for your cheque cashed.
The key point is trusting the sending bank. If you recall, this was where the economy almost broke down completely in 2008 - banks would not advance money if they could not be sure that sending the request to the corresponding bank would not result in re-payment, with so many banks crashing. One bank I dealt with (in Canada) had a feature where money was instantly available - to trusted customers. i.e. you did enough business with them and pushed through enough money for long enough, were a long term good customer - so they trusted you were not trying to pull a fast one.