I think it should be done via email. Monzo could have receipts@monzo.com alias set up for this purposes.
User would forward an email with receipt and Monzo should be able to automatically match it, various factors should make it quite definitive: by price, merchant name and lastly timedate stamp to a transaction. Failure rate should be really low, but it would be good to allow user to match it manually if there’s a need.
Degrading rich data in an email into bunch of pixels is absolutely barbaric.
Monzo can archive email for us and ideally, even allow us to print it when the need arises. Archiving an email also takes up less space than archiving a photo.