Moving a picture box

I have an array of picture boxes on my form. Some elements of the array are empty. That is a picture box with no picture. I want to move one of the pictures, with the mouse, to an empty box. The pace it came from will be an empty picture box. I also want the picture I am moving to snap into the empty picture box. I will worry about the rules to allow the picture to go there.

Any help would be appreciated.

Thanks