Use scanlines to give your image an interesting horizontal finish. It’s best to use this effect on lighter images, as it adds quite dark tones. For more subtle effects use smaller images, like my example below.
Press D on your keyboard. Then create a new image 1 x 2 (pixels) with a transparent background.
Press Ctrl and hold down whilst you press + to zoom in, until the the tiny white box in the bottom left of your screen says 1600%.
Click the pencil tool. Set it to 1 and click the bottom of your image:
Next, go to Edit > Define Pattern and click Ok.
Now open the image you want to add scanlines to.
Go to Layer > New > Layer and click Ok.
Now click the floodfill bucket icon in the tool menu. At the top of the screen choose ‘Pattern’ in the drop down box. Then floodfill your image.
Go to Layer > Layer Style > Blending Options, and change the opacity to get the effect you want. Then press Ok.
Finally go to Layer > Flatten Image, and then save.