One of the ways that Photoshop can help blend multiple images together is through using the “Blend If” sliders in the Blending Options of the Layer Styles dialog. In this example I want to blend the clouds from the first image into the sky of the rock image.
With the Cloud image selected on the Layers panel, I choose layer > Layer Style > Blending Options (or, you can use the fx icon at the bottom of the Layers panel and choose Blending Options…).
In the Layer Styles dialog, I moved the black slider for the Underlying Layer to the right to hide the dark foreground values of the cloud image. In order to create a smooth transition, Option + (Mac) | Alt + (Win) click and drag the black triangle to split it into two. The values to the left of the split triangle will be completely transparent the values between the split triangle will transition from transparent to opaque, and the values to the right of the second split triangle will be fully opaque.
It’s OK if the rock in the foreground is semi transparent at this point – you want to focus on the “transitional areas” – where the new sky (the clouds) will meet the ocean and the top of the rocks.
To bring back the solid rocks in the foreground, I made a copy of the rock layer and moved it above the new sky (the clouds) layer in the Layers panel. Then, I added a layer mask and painted with black to hide the drab sky and reveal the clouds below, while keeping the rocks solid.
Because this example has a fairly straightforward horizon to mask, you might feel that I’m making this process or technique overly complicated. However, the Blend if sliders can be tremendously useful when masking detailed objects such as a tree against a sky. Notice that you can even change the Blend If options to blend individual color channels.