How to Use a Gradient Mask in Adobe Photoshop
Use a gradient mask in Photoshop to help blend one image into another. The depth and opacity of the blending may be adjusted as well as the blending mode of the layer.
This feature works the same in all recent versions of Adobe Photoshop: CS5, CS6, and Creative Cloud (CC).
- Open two images. In this example, the first image is a heron and the second image is a water scene.
- In the first file, select the Move Tool, then click and hold on the image and drag it to the second file.
- Position the heron in the lower right corner of the water scene so that the top edge of the heron image aligns with the far shoreline in the water's background. Then, using the Rectangular Marquee Tool, drag out a rectangle around just the heron image.
- Go to the Layers Panel and click the Add Layer Mask button  at the bottom of the panel.
- Select the Gradient Tool . Make sure the foreground and background colors are set to white and black.
- Position your cursor in the image so that it is level with the tip of the heron's bill. Hold down the Shift key, then click, hold, and drag right to left to the edge of the layer. Release.