With the photo in a layer (I don't believe that the background layer will work for this.) and the layer selected, you'll find an icon in the upper right corner of the layers toolbar. Clicking on it will open a pane where you can enter the layer's opacity on a 0% to 100% basis.
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With the photo in a layer (I don't believe that the background layer will work for this.) and the layer selected, you'll find an icon in the upper right corner of the layers toolbar. Clicking on it will open a pane where you can enter the layer's opacity on a 0% to 100% basis.
In the Layers tool window right-click on the layer with your photo and select Blending Options. Set Opacity to the level you like.