how to divide a map (pgm) in little cell

I want to code a complete coverage path planner. So first I got a map, and I want to know how can I divide my map in little square cell and in the middle of each cell put a point. With visualization_msg, the point can just be seen by us, and not by the robot. What I want is that this point can be seen by it. So then I will be able to tell to my robot to get throw all the mark. Thank you