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A common reason for combining two photographs is to replace a gray sky with a blue sky.
Here, for fun, a blue sky is being replaced with the YUMMY Buffet restaurant.
Be sure to check off as you go along.
1) Open the photograph with the blue sky.
2) Open the YUMMY Buffet restaurant photograph.
3) Transfer the YUMMY Buffet restaurant photograph layer to the blue-sky photograph.
If you haven't already done so, go to Move a Layer.
Make sure the YUMMY Buffet restaurant layer is selected (Layer 1).
Brush on the image where you want the blue-sky photograph to be revealed.
If you haven't already done so, go to Brushes - Basics.
You can see the erasure below in the YUMMY Buffet restaurant layer.
Here's the combined version, along with a version in which a cooling filter made the YUMMY Buffet "sky" more blue.
1) To add blue to the YUMMY BUFFET restaurant layer, create a Photo Filter adjustment layer.
Click the Create adjustment layer icon, the half-dark, half-bright, circle at the top of the Layers palette, and select Photo Filter.
The Photo Filter window will open.
The default density, 25%, was used here.
Make sure Preserve Luminosity, the default, is selected.

3) Group the Photo Filter layer with the YUMMY BUFFET restaurant layer (Layer 1).
If you haven't already done so, go to Grouping.

If you haven't already done so, go to Saving Files.