Have you ever wondered how to make an image with those diagonal lines on? Well, they are called scanlines and it's alot simpler to do than you might have first thought - presumably your first thought was that they were painstakingly drawn on by hand.
They're actually done using a pattern and it's really simple to make.
Step 1
Open up a new page with a size of 5px by 5px. Now, on the layers tab, right click the layer and click Add Alpha Channel - this allows your background layer to have transparency.
Using either the wand tool or the colour select tool, select all of the image, which should be all the same colour, most likely white. Remove it by pressing Ctrl-X. You are now ready to make the pattern.
With the pencil tool, and setting the brush as the smallest possible - 1px by 1px, draw a line pixel by pixel from one corner, diagonally, the opposite. I told you it was simple to make!
Step 2
Now, the tricky bit is saving it so you can use it. Begin by saving as you would normally, however, instead of saving it in the usual folder, select GIMP's patterns folder - My Computer > Program Files > GIMP 2.0 > share > GIMP > 2.0 > patterns. You should find various other patterns as "3dgreen.pat" and "corkboard.pat", just hunt for them if you're having trouble finding it.
Once you have the patterns folder selected, just type in the name you want to give this pattern - probably something like "scanlines"; add the .pat extension. Once you click save, simply restart GIMP and it will be ready to use!