check max images, use copy

config.go

@@ -144,6 +144,10 @@ func SanitiyCheck(conf *Config) error {
 		return fmt.Errorf("at least 1 image must be specified")
 	}
 
+	if len(conf.Image) > 4 {
+		return fmt.Errorf("only 4 images can be specified")
+	}
+
 	if conf.Shader == "" {
 		return fmt.Errorf("shader file must be specified")
 	}

game.go

@@ -141,9 +141,7 @@ func (game *Game) Draw(screen *ebiten.Image) {
 		"Mouse":  []float64{float64(mousex), float64(mousey)},
 	}
 
-	for idx, image := range game.Images {
-		op.Images[idx] = image
-	}
+	copy(op.Images[:3], game.Images)
 
 	op.GeoM.Translate(float64(game.Conf.X), float64(game.Conf.Y))