Attaching an image or a PDF to transaction is easy.
Make sure that the attachments field is enabled (it is enabled by default but you may have disabled it at some point). To do that go to MoneyWiz / Settings / New Transaction Screen. Then make sure that the "Attach images" field is enabled.
Go to the new transaction screen (i.e. the screen you see when creating a new transaction). By default. the attachments field is hidden as it's in the extended fields section. To make it show, tap on the "Show All ▼" button.
Now tap on the "+" sign of the attachments field and select the file you want to attach.
Additionally, you can enable attachments sync between devices. To learn how, check out this article: How to sync transaction attachments