You can forward images and pdf receipts from your inbox to Billy using your unique email forwarding email.
Once the email is sent and the bill is created in Billy, you can now categorize the expense and match it with any bank transaction.
First, visit your Settings, then go to Receipt Capture Tools. Here is where you'll find the unique forwarding address to your organization in Billy.
I would suggest adding this long email as a contact in your email program and give it an easy name like, "Billy Receipt Forwarding" so you can just type that the next time you need to forward on a receipt.
Note: the page also contains several other receipt capture methods that can be used.