An image reference tool similar to ZBrushe's "Spotlight"

I don't care about the extra features from Spotlight such as project, but what that tool is really useful is for providing a constant reference image that doesn't move with the camera or perspective view, like a pop-up window that you can move around and resize at will. It is extremely useful for references when you don't have a blueprint of the model, but just a concept that you need to sculpt from, so you are constantly looking at it while modeling and sculpting.

Image for reference attached.