-Make a new file 2 pixels by 2 pixels with transparent background and zoom in all the way.
-Using the pencil tool, fill in the bottom with black.
-Go to edit>define pattern and name it scanlines.
-To use the scanlines on an image, go to Layer>Layer Style>Pattern Overlay, choose your scanlines patterns, and set the opacity to whatever looks good.
(This is for Photoshop. If you have Paint Shop Pro, get rid of it and get Photoshop.)