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Layer Mask from Grayscale Image
By Cari Jansen | July 25, 2004
Digitip 025 - Adobe Photoshop CS
(revision 1.1)
During a Saturday training session, one of the trainees asked a question about creating a layer mask based on an existing greyscale image. A solution was found, that seemed too much like a work around. The following tip uses a copy-’n-paste approach.
The images
Start by opening both images. The image that will be masked and the greyscale image.
The destination image.
The greyscale image (grayscale if you’re in the U.S.).
If your image is inversed then use the Image> Adjustment>Inverse command to swap the black and white colours around.
Converting Background to Layer.
Next, convert the Background into a layer by double clicking the Background icon. The New Layer dialog box will appear.
Either enter a new layer name, or keep the default “Layer 0″. Don’t amend any other settings. Click OK to convert the Background to Layer 0.
Creating a layer mask.
With Layer 0 selected, click the “new layer mask” button at the bottom of the layers palette . A blank mask is added to the image. Click the mask icon in the layers palette to select the mask. Then access the channels palette. The mask channel is automatically selected.
With the layer mask selected click the view mask icon, then paste the copied greyscale image. (Edit > Paste).
To view the image without the “red” maske overlay deselect the view Layer 0 Mask icon again.
The finished result provides transparency for black maske areas and semi transparency (opacity) for grey areas in the mask. Use the painting tools to expand the mask if required (when placing over larger image files), and move the mask to preferred position over the image (remember to UNLINK first when doing this, as you’d be moving both image and mask simultaneously otherwise.)
Save the file as a native Photoshop file (retain layers) then place it into your favourite page layout appliation … InDesign CS 
Ta-daaaaaah!
There are a number of methods to create the above generated affect, and this tip merely outlines one of the many methods available in Photoshop. Thanks for asking the question Josh!