1. create 2 pixel high, 1 pixel wide canvas
2. colour the top pixel black, the lower one grey
3. export to gif
and if you want to use it in photoshop, and in a design you are working on, you should make a pattern. In that case, skip step 3 above and:
4. make a selection of the whole canvas (2x1 pixel that is)
5. Go to edit > define pattern
6. Create a new layer in your design
7. Go to edit > fill > pattern
finished
