Scanlines are fairly easy to make. Here is how to make a basic scanline.
Step One - New Document
Open Photoshop. Go to File > New and make sure you have the following before clicking OK:
Width: 3px
Height: 3px
Resolution: 72px/inch
Color Mode: RGB Color 8 bit
Background Contents: Transparent
Step Two - Make it!

Zoom in to the max - At the bottom left hand corner there is a number and a percentage. Type in 1600% and hit enter. Select the Paint Bucket Tool and fill in your square black. Use the select tool to cut out this shape.
Step Three - Save it!
Go to Edit > Define Pattern and OK.
Step Three - Use it!

Open a picture file (I will use the Example One sig) and make a new layer, placing it at the top. Go to Edit > Fill > Use: Pattern and select the pattern we just saved. Under Layers select "Overlay" instead of "Normal" and then change the Opacity to about 20%. Voila! Scanline complete.