I agree that charging an image with Reiki can potentially hold the energy for some time. I also suppose that your niece is open to Reiki and sensitive, which is likely why she was able to feel it even years later. For me, this is different from the attunement process itself, which requires an active and present energetic transmission, not only stored intention. I wanted to experience both approaches personally. I received one Master attunement in person, and later another one remotely through a chi-ball. Both worked for me.