With that script call, you can change the opacity of the picture, without using the game show/move pictures.
But to do the slow fade in/out you have to do a bit more eventing. Maybe use a variable to keep track of the time you want to the fade happen.
Like variable duration = 60;
$gameScreen.picture(id)._opacity -= (255/duration);
if(duration <=0) break loop
Something like that, I'm not entirely sure about the math here xD
But maybe you can find your way out