Google Drive is Google's file sync app that lets you store all of your files online alongside your Google Docs documents, and keep them synced with all of your devices.
Call out a changed proof before you confirm the creative decision.
Keep reviewers aware of changes to a proof before they act on a decision. Your task shows the mismatch and asks for confirmation.
Keep creative signoffs tied to their exact proof uploads.
Keep a clear history of who approved or rejected each campaign proof, even when the review board status changes later.