The idea of intentionally visualizing someone's healing before it actually occurs strikes us as being very similar to the psychological technique called 'visualization.' This is not a biblical method. We would agree that God may sometimes show people something prophetically before it happens, but to try to conjur up the image in order to make it happen is an ungodly technique.
We cannot say why some people do certain things or why some people get healed instantly, some over time, and some not at all. What we can say is that Jesus Christ is fully able to heal anything. There is nothing too hard for the Lord!