how superimpose a moving picture over an axis?

I have an axis on which I have to put some objects (pictures) on it to create a scenario.

I want to do it choosing the object picture by clicking on it and dragging it into the axes in order to put the object in the desired position.

During the moving I need that the object background not covers the other objects present on the axis. How can I make the object background transparent during the moving?

Thanks