open photoshop
File -> New with the dimensions: 3x3 pixels (make sure the background is transparent)
Click OK
create a diagonal line, when finished click Edit -> Define Pattern
name it whatever you want (scanline would be easier to find it :p)
click
open the sig/piece-'o-art you want the scanlines on
make a selection of where you want it, go to edit -> fill
a menu will pop-up, choose Pattern in the drop-down menu
and choose your freshly made scanline and press OK
click
fiddle a bit with the layerstyles and fill-settings ;)
(on your sig, I put blending mode on soft-light