Be certain the image layer is active and add a mask to that layer, Layer – Add Layer Mask.
Select the Gradient Tool from the Toolbox and in the Gradients pallet, choose a black-to-white gradient. Adjust the blending slider (top) to 80%.
With the Gradient Tool, drag from the bottom left corner toward the center of the image.
Below is another example of the technique in which I used a radial gradient on the mask rather than a linear one.
– Thomas Boito