this tutorial is for photoshop
1. Make a new image that is 40 pixels by 2 pixels with a transparent backround.
click ok. it should look like so:
2. It is extremely small so make sure you zoom
in so you can actually see what your doing. Find the Rectangular Marquee
Tool (or as i like to call it the select tool) 
and
select the top half of the image 
3. Now use the fill tool to
color in the selected part of the image black. then click select>>decelect. 
it should look like this
now.
4. Then go to Edit>>Defign pattern... a window should pop up. Title it "Scan Lines" or something simular. and click ok.
5. Now open the image that you want the scan lines to be on. Go to layer>>new layer
6.click
on the fill tool again and look at the upper left hand corner of the screen.
there should be a bar there. make it look like this: 
you
can change the opacity to whatever you like,
but in this picture i chose 65%.
7. now fill your picture. go to layer>>flatten image. your done! cool huh?
Email me if your have any questions