Put an image on top of a background?

This may be a stupid question but I can't figure out how to put an image on top of a background and be able to fade that in and out.
I want this:


But I can only figure out how to do this:

Where the background is always visible through the circle.
Am I missing a node or is it more complex than I initially thought?
I want this:


But I can only figure out how to do this:

Where the background is always visible through the circle.
Am I missing a node or is it more complex than I initially thought?