HarmonyOS如何使用Image占位图

@Entry 
@Component 
struct xxxx { 
  @State message: string = 'Hello World'; 
  @State altImage: string | Resource = $r('app.media.startIcon') 
  build() { 
    Row() { 
      Column() { 
        Image('') 
          .width(100) 
          .alt(this.altImage) 
          .margin(20) 
 
        Image('gfdsgfdg') 
          .width(300) 
          .alt(this.altImage) 
      } 
      .width('100%') 
    } 
    .height('100%') 
  } 
}