To move an image from one document to another, you will first need to unassociate the existing image from the existing document. The document will then go back into the Inbox for re-processing.
To unassociate a page from a document:

Click the Inbox tab. Click History. This displays the Files History table, which lists every document you've sent into Bill.com and processed.
Click the name of the file of whose page(s) you wish to unassociate. This image of the page of this document will now display, along with the This page is: box.
Use the forward and back arrows to navigate to the page you wish to unassociate.
When you reach this page, in the This page is: box, click Unassociate.
The unassociated page has been returned to your Bill.com Inbox and can be tagged again as a bill, a vendor or a company document.