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.
Your campaign team finds the current draft on its review card, with a version note to make the handoff clear.
Your reviewer gets the current asset with a clear request, and your campaign task keeps the handoff in view.
Your review task shows file access problems before review starts. The owner sees what to fix.