I began this process with a simple portrait I took in the studio. Only one side of the models face is lit.

My intentions are simple, to add a cityscape to the dark side of the image.

The image I will add must be quite faint, I will decrease the opacity to around 60/70%.

The image on the left is what I am layering on top.

As you can see I have made it very opaque.

When I add it onto the other image it should clearly and subtly cover the other side of the image.