Drag the mask layer below the layer containing the photo or artwork in the Layers panel. If your photo is a Background layer, first create a copy by dragging the "Background" layer onto the "New Layer" button at the bottom of the Layers panel. Drag the "Background Layer" into the trash or hide it by clicking the "Eye" beside its thumbnail in the Layers panel. You can now drag the mask layer below the "Background Copy" layer.
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