Welcome ^_^ People normally use screenwidth and screenheight to place custom status bars on screen. Like:
NPC Code:
if (timeout) {
showimg 1000,myhpbar.gif,screenwidth*0.85,screenheight*0.1 ;
changeimgpart 1000,0,0,int(playerhearts/playerfullhearts),10;
timeout=0.05;
}