How can I move a black & white image to a layer mask in Photoshop?
My image includes shadows, lights, etc and I can't just select area and paste to mask.
I need this saved in grayscale.
Add a Layer Mask
In the Channels Panel highlight the layer mask channel and turn off visibility for the other channels. (And turn on visibility for the mask channel)
You can then turn visibility back on for the other channels, turn visibility off for the mask channel, and you have a layer mask of your greyscale image.
You can also merely create a New Alpha Channel then paste. You won't have a mask this way, but you will have your greyscale image on a channel, which you can then use to create a selection and subsequently a mask.
*Note: You do better by preparing the BW image to fit into the mask excactly without moving. You can for example paste it at first as an ordinary layer and place it there. You can see through it by reducing the layer opacity. When your new mask candidate is ready, copy it to the mask (=paste in place) and disable or remove the temporary layer.