要将Flutter App中的页面设置为全屏显示，可以在Scaffold的构造函数中添加如下代码：

Scaffold(
  body: Container(
    decoration: BoxDecoration(
      image: DecorationImage(
        image: AssetImage("assets/images/bg_image.png"),
        fit: BoxFit.cover,
      ),
    ),
    child: Center(
      child: Text(
        "This is a full-screen page",
        style: TextStyle(
          fontSize: 24,
          fontWeight: FontWeight.bold,
          color: Colors.white,
        ),
      ),
    ),
  ),
);

其中，Container的decoration属性可以设置图片框。在本例中，我们定义了一张名为"bg_image.png"的背景图片，在Container的decoration中引入，并将其铺满整个容器。

可以使用如下代码在Flutter App中实现将页面设置为全屏显示：

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Full-Screen Page',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        body: Container(
          decoration: BoxDecoration(
            image: DecorationImage(
              image: AssetImage("assets/images/bg_image.png"),
              fit: BoxFit.cover,
            ),
          ),
          child: Center(
            child: Text(
              "This is a full-screen page",
              style: TextStyle(
                fontSize: 24,
                fontWeight: FontWeight.bold,
                color: Colors.white,
              ),
            ),
          ),
        ),
      ),
    );
  }
}