1. We are using this world image for this tutorial. You could use this technique on any image.
2. Open your image in photoshop.
3. Now for the pattern you need create a new document (transparent background). Select the Pencil Tool.
Using the Pencil Tool, make a small 2x2 pixel box. We suggest you zoom in for the perfect pixel picture.
Now select the Rectangular Marquee Tool. Select the box plus 1 pixel on the right and 1 on the bottom. It should look like the picture below.
4. Click on Edit > Define Pattern, name it and save it.
Create a new layer.
Click on Ok. Go and delete your original blue world and you should have something that looks like the image below.
Pretty easy eh.