How to make canvas to response on dragging things on

I need to make canvas change its images once the outer image was dragged and dropped on top of it. Now it only works with dragging and dropping things on top of it but without any effect to the canvas so it could change it's white images to blue ones along the road to the end when an imager dropped on top of it. I'd appreciate some working examples.
Here comes the ideal working example in the attached files.
Source code -> https://ift.tt/35tBJrP
Source images ->...

How to make canvas to response on dragging things on

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