Here is the original background image.

A stock photo of this skull was taken and clipped out with the Magic Wand Tool. Name this layer “skull”.

Create a new layer and place it below the skull layer. Name this “skullcloud”. choose black and white as the foreground and background colors. Go to the top menu and navigate to Filter>Render>Clouds.

Go back to the skull layer and magic wand the area outside the skull. Press Ctrl+Shift+I to invert the selection which will create a selection in the shape of the skull. Go back to the skull cloud layer and click it to make that active. Go to the top menu and use Select>Modify>Feather and type 25 pixels. Press Delete. A nice soft skull shaped cloud should remain.

Activate the top skull layer and navigate to Filter>Blur> Gaussian blur. These are the setting that I used.

Here is what the layers panel will look like when the following is completed. Choose the top skull layer and set the blending mode to Soft Light. This will reveal the clouds below. Then double click the skullcloud layer and choose the Color Overlay Layer Style.

Click on the color swatch and an eyedropper will appear for you to select a color from the background source image. I picked one of the darker blues.

Transform, rotate and scale to taste and you’re on your way to some spooky imagery.