This is a minimal file to demonstrate a grid.
Note that the image file is just a 40 by 40 pixel icon with just the top row of pixels and the left row of pixels colored. When the image is tiled, it gives the appearance of the grid filling the browser window.
All the pixels in the top row of the image are filled solid with color, giving a strong horizontal line. The left column of pixels is filled every fourth pixel, giving a lighter vertical line in the grid.
Note that you'd want to use fairly light colors for a grid in the background image.