Well, there are two ways:
1) Find an action that will do it. There are a lot of them.
2) Create an image that is two pixels tall and one wide. Then place a dark color in the top pixel and a light color in the bottom pixel. Make a new pattern from that image, and then create a new layer in your other image and fill from pattern. Then change the transparency of that layer as it suits you.
Hope that helps!
R51
