problems with setimgpart and y-=;

[Vima]

// NPC made by SwedishKing
//#CLIENTSIDE
function onCreated() {
show();
setimgpart treetop.png,0,0,33,180;
y-=0.3;
}
function onPlayerTouchsMe() {
dontblock();
drawunderplayer();
}
The script above works, but I just want help to understand why I cant use
setimgpart(treetop.png,0,0,33,180);
instead of
setimgpart treetop.png,0,0,33,180;

Because when I do it, then the script doesnt work Online :/
Does anyone know why it doesn't work?/ and how do I "fix" it?

btw: Whats y-=0.3; in GS2? Is it the same? (Im currently learning how to script GS2 :P)