2.8 Eevee Composite Object over background lacking Transparency

Can anyone provide instructions on how to composite my object over an image background in 2.8, Eevee render? The problem is that the object is does not have transparency around it. I set a bounding box but that only shrunk the background. I’ve looked for transparency settings in the the shader and in the renderer and in the compositor but am not finding anything. I’ve tried both Alpha over and Mix. See attached screenshot. I want just that green cube floating in the air near the dude’s head (just experimenting), minus the dark gray around the green cube.