How to make an image and canvas able to use the same grid?

Here is how I understand your question:

If you want your canvas and your image share the same grid (a common size of the individual squares), that simply means the size of the square will be the same unit used by both the canvas and the image to overlay. The 'squares' are the 'units', why not use the same unit, centimeter or inch as the common factor? So in the preferences set the rulers and grids to the same fixed unit, cm or in.

Of course, that means that you know both the size of the canvas and the image measured in cm or inches.

So, open both your canvas and your image in the photo bin.

The drag and drop move of the image from the photo bin to the canvas should not be used there because the image preview of the 'smart layer' would fill up the canvas.

Instead, either use a copy and paste of the image to the canvas, or the 'place' command, or even dragging a layer from the opened image to the canvas.

Then the squares will be the same dimension in both your image and the canvas, and the move tool can position the image where you want, with the available 'snap to grid' default.