If you realize that you need to add an asset while in the session, simply open another browser tab and go to the My Sessions page and choose Edit Session > Session Assets.
Upload new assets, by choosing Add New Asset or add existing assets from the Asset Library. (See complimentary videos and help articles on uploading assets)
Once you’ve added the asset(s), return to the session and click the Refresh icon found in the Presenter Toolbar. This will add the new asset into the session.