This has come up before, but i haven't quite grasped the value of it entirely until this post.
In the past when this has come up i have suggested that people simply use a standalone image viewer, of which there are many (irfanview is the one that comes to mind).
But I can definitely see the value of a simple quick image overlay that you could drag around and would stay on top, perhaps with transparency controls.
Now, I *could* fairly easily add such a function directly to Screenshot Captor, but I think this would be a perfect candidate for a standalone little tool that someone could write for NANY 2014
. It's trivial to interface such tools into Screenshot Captor.