Milestone 2014 R3
Lookup SetOverlay on CImageViewerInterface Class Reference
It is important to note that the ImageViewer ActiveX is very close to end of life..
http://doc.developer.milestonesys.com/html/index.html?base=ivaxhelp/main.html&tree=tree_2.html
I would strongly recommend that you switch to use the ImageViewerControl (or ImageViewerWpfControl) instead. Using this there are samples on how to do overlay.
The Video Viewer Quality sample uses overlay.