怎么把本地图片当作小程序背景

<view class="root">
  <image class=&#39;background-image&#39; src=&#39;../res/login_bg.png&#39; mode="aspectFill"></image>
  <view class="content">
  </view>
</view>

.root {
   width: 100%;
    height: 100%;
    background-color: #70c7da;
    position: relative;
}

.background-image{
   height : 100%;
    position: absolute;
    width: 100%;
    left: 0px;
    top: 0px;
}
.content{
    position: absolute;
    width: 100%;
    height: 100%;
    left: 0px;
    top: 0px;
}