C# wpf image绑定viewModel没有显示图片

在wpf绑定图片，用viewModel的图片绑定image
我是用viewModel.cs

public class viewModel:INotifyPropertyChanged
{
        #region
        ///代码
        #endregion
        public event PropertyChangedEventHandler PropertyChanged;

        public BitmapImage img
        {
            set
            {
                _img= value;
                OnPropertyChanged("img");
            }
            get
            {
                return _img;
            }
        }

        private BitmapImage _img=new BitmapImage(new Uri("imgf/img.jpg" , UriKind.Relative));
        //UriKind.Relative 相对

        protected void OnPropertyChanged(string name)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                handler(this , new PropertyChangedEventArgs(name));
            }
        }

}

<Image x:Name="image" Source="{Binding Path=img}" Width="300" Height="auto" HorizontalAlignment="Center"/>

结果不会显示图，debug发现

<Image x:Name="image" Source="imgf/img.jpg" Width="300" Height="auto" HorizontalAlignment="Center"/>

显示

                BitmapImage t=new BitmapImage(new Uri("imgf/img.jpg" , UriKind.Relative));
            image.Source = t;

显示

image.Source=viewModel.g_viewModel().img;

不显示

想到图片位置错误，检查。

用imgf/img.jpg可以绑定image

ImageSource 和 BitmapImage可以绑定image

操作viewModel img可以xaml image