Preloader - When should I use one, how and why?

programming
1

I think I might need a preloader as I load in tilesets. My problem is that I have no idea how to do this. The examples I have seen so far preload swf files.
I don’t think that that is what I need as I don’t see how that preloads all png files that my game uses, for example this class:


package nl.obg.www {
    import flash.display.Loader;
    import flash.display.Sprite;
    import flash.net.URLRequest;
    
    public class Flag extends Sprite {
        private var tileSize:uint;
        
        public var tx:int;
        public var ty:int;
        
        private var _loader:Loader;
        
        public function Flag(nwX:int, nwY:int, nwZ:int){
            x = nwX;
            y = nwY;
            z = nwZ;
            
            _loader = new Loader();
            addChild(_loader);
            _loader.load(new URLRequest("../images/flag.png"));
        }
        
        public function init():void {
            tileSize = (parent as Map).getTileSize();
            
            tx = x;
            ty = y;
            
            x = x*tileSize;
            y = y*tileSize;
        }
    }
}

How would one preload this png and others?