1. Make a new document, size 8pixel by 8pixel.
2. Next, Zoom in all the way (to 1600%). Choose the pencil tool set to 1 pixel, color black. Then fill in the first row and first column.
3. Go to Select -> All. Then go to Edit -> Define Pattern to set it as a pattern. Name it "Gridlines".
4. Now, open the image you want to add the gridlines to. Go to Edit -> Fill and choose Pattern from the dropdown. Then choose the Grid pattern.
Your final product will have a grid over it like this.
To lower the opacity of the grid, after you fill the ficture, go to Edit -> Fade Fill and lower the opacity settings to your likings.
