1-Upload an image in 'Global' site
2-Setup global with remote staging
3-Publish to Live in Global site, and ensure the new image is in remote server.
4-Add a Site and setup to be remotely staged.
5-In that site, add a content (Web content or Blog entry) with a link to the global document.
6-Publish to Live that Site.
You'll see the link is pointing to something like this instead of document: