title: draw grid with cells of specific size over an image
body: <p>How can I draw a grid, 1px black borders no fill, over an image?</p>
  
  <p>Each section of the grid should be 480x360px.</p>
  
  <p>E.g., given this 1440x1080px solid white input:</p>
  
  <p><a href="https://i.stack.imgur.com/qLzc1.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/qLzc1.png" alt="enter image description here"></a></p>
  
  <p>It should have a 3x3 grid drawn onto it (because 9 480x360px rectangles fit into it) to make an output kind of like this:</p>
  
  <p><a href="https://i.stack.imgur.com/X23PE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/X23PE.png" alt="enter image description here"></a></p>
  
  <p>That's not as accurate as I'd like the command to be (I was just drawing rectangles by eye), but I hope it illustrates what I'm after.</p>