Graphic artists have long used the grid method to ensure they get the proportions right when copying or enlarging an image from a photo. Once you add a grid to a photo, it's easier to accurately reproduce the image by creating an equally proportioned grid on canvas or drawing paper and copying the image on a square-by-square basis. Instead of printing out a digital photo and adding grid lines by hand, a graphics editor like Photoshop or a specialized artist grid app can simplify the process by letting you add grid lines directly to a digital image.
Grid Pictures in Photoshop
Before adding a grid to a photo on your computer, it's always a good idea to save a copy. You can do this by copying and pasting the file in its folder or by loading it into Photoshop and using the Save As command. In Photoshop, you can use the Layers menu to add a new layer to the photo image.
Make sure you have rulers displayed to help space your grid lines, then use the line drawing tool to create a series of straight grid lines. Choose a grid spacing that makes sense for the size of your image and use a line color that will show up over the image. Once you have a few horizontal or vertical lines, use copy and paste to duplicate them until the grid is filled. Repeat for lines in the other direction.
Some artists add reference numbers to grid spaces in one or more columns and rows to help them quickly find a specific square while copying. You can do this with the Photoshop Text tool. You probably don't want to number every square since it could hide or obscure too many image details.
Try numbering the spaces in the outer rows and columns and possibly a row and column in the center if your grid is large. When you're done drawing the grid, use the Flatten Image command on the Layers menu to combine the layers and then save the image as a JPEG file that you can print or send to a photo processor. You can also display the image on your computer monitor and copy it from there.
Create a Grid in Microsoft Paint
If you don't have access to Photoshop, a simple graphics editor like Microsoft Paint can be used to add a grid to a digital image. There's no concept of layers in the Paint application, so you'll need to create lines directly on the photo image. Turn on ruler display from the View tab and use the line drawing tool, with the line width set as thin as possible to avoid obscuring image details.
Use an Artist Grid App
Another option for adding a grid to a photo is downloading an app that's specifically designed for this purpose. Searching for "proportion grid maker" or "artist grid" in the Apple or Google app store shows dozens of grid creation tools. The advantage of using one of these dedicated tools is that you won't have to draw the grid lines.
Apps like Drawing Grid Maker allow you to enter the number of rows and columns you want and set the line width and color. The grid is then automatically generated on the photo to be printed out or displayed on your monitor, phone or tablet while you copy it. You can easily change the grid by altering its settings instead of having to redraw it.
Put a Grid on a Photo Print
Drawing a grid by hand on a print photo can be difficult, especially if it's 5 by 7 or smaller in size. If you first scan the print into your computer, you can add a grid just as you would to any other digital image. Be sure to use a scan resolution of at least 300 dpi to avoid blurring. Use 600 dpi if you don't want to lose any detail from the photo.