let $this = this;
request({
   url:'https://www.dounine.com/hello.jpg',
   method:'GET',
   responseType: 'arraybuffer',
   success:function(res){
    let base64 = wx.arrayBufferToBase64(res);
    $this.data.userImageBase64 = 'data:image/jpg;base64,' + base64;;
   }
});

<image src='{{userImageBase64}}' style='width:90rpx;height:90rpx;' />

wx.chooseImage({
 success: function(res) {
  console.log(wx.getFileSystemManager().readFileSync(res.tempFilePaths[0], "base64"))
 },
})