A proof of payment can be a receipt (either a scan, a photo or a PDF) or a screenshot from your online bank, clearly showing the following:
We don't always need proof when you make a transfer. If we do, we'll ask you to upload it, or to send it to us as an email attachment.
If you aren't able to fit all of this information into one image, you can take 2 screenshots. They both need to show your bank account number to confirm the information is for the same account.