Generate a checkered grid pattern (check, checker, chequer) as image for camera calibration or as a game board (e.g. chess, draughts/checkers/chequers, go).
Note that these are generated as raster image so the real-world dimensions depend on the output device's resolution (display, printer).
Save it via your browser's context menu's (e.g. right mouse button click) "Save Image As..." function.