Size of image corresponds roughly to the sounds duration.
Opacity corresponds to active (solid) or passive (transparent) use of the object - for example when objects are simply additionally resonating.
The image of my hand is used whenever I exclusively use my hand, without any objects, for example to pluck or rub the piano strings.
The image of the keys is used when they are in any way part of the sound producing process.
This score is completely ideosyncratic and without any additional explaining legend, it is meant as a study.