Start with a silhouette.

Your silhouette photo should be high contrast with just a little bit of detail showing through.

Add a fill image.

Select another photo — landscapes work well — and make it a new layer above the silhouette layer.

Blend the layers.

Select the fill image layer and set its Blend Mode to Screen.

Adjust if necessary.

To finesse the shadows, highlights, and midpoint, create a Levels Adjustment Layer. Double-tap it and move the sliders to see how it affects the details in the silhouette.