WPF show ImageSource via MVVM and timer

//xaml
<Window x:Class="WpfApp45.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        WindowState="Maximized"
        xmlns:local="clr-namespace:WpfApp45"
        mc:Ignorable="d"
        Title="{Binding ISBN,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}">
    <Window.DataContext>
        <local:BookVM/>
    </Window.DataContext>
    <Border>
        <Border.Background>
            <ImageBrush ImageSource="{Binding ImgSource,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                        Stretch="Uniform"
                        RenderOptions.BitmapScalingMode="Fant"/>
        </Border.Background>
        <TextBlock Text="{Binding ISBN,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                   HorizontalAlignment="Center"
                   VerticalAlignment="Center"
                   FontSize="200"
                   Foreground="Red"/>
    </Border>
</Window>


//cs
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.IO;

namespace WpfApp45
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }

    public class BookVM : INotifyPropertyChanged
    {
        public BookVM()
        {
            InitData();
            InitTimers();
        }

        private void InitTimers()
        {
            System.Timers.Timer tmr = new System.Timers.Timer();
            tmr.Elapsed += Tmr_Elapsed;
            tmr.Interval = 500;
            tmr.Start();
        }

        private void Tmr_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
            if(++ImgIdx>=imgsCount)
            {
                ImgIdx = 0;
            }
            ImgSource=GetImagesSource(imgsList[ImgIdx]);
            ISBN = $"ISBN_{++calcIdx}";
        }

        private void InitData()
        {
            imgsList = new List<string>(Directory.GetFiles(@"../../Images"));
            if(imgsList.Any())
            {
                imgsCount = imgsList.Count;
                
                
                ImgSource = GetImagesSource(imgsList[imgIdx]);
            }
        }

        private ImageSource GetImagesSource(string imgUrl)
        {
            BitmapImage bmi = new BitmapImage();
            bmi.BeginInit();
            bmi.UriSource = new Uri(imgUrl, UriKind.RelativeOrAbsolute);
            bmi.EndInit();
            if(bmi.CanFreeze)
            {
                bmi.Freeze();
            }
            return bmi;
        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged(string propertyName)
        {
            var handler = PropertyChanged;
            if (handler != null)
            {
                handler?.Invoke(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        private ImageSource imgSource;
        public ImageSource ImgSource
        {
            get
            {
                return imgSource;
            }
            set
            {
                if(value!=imgSource)
                {
                    imgSource = value;
                    OnPropertyChanged(nameof(ImgSource));
                }
            }
        }

        private int imgIdx;
        public int ImgIdx
        {
            get
            {
                return imgIdx;
            }
            set
            {
                if(value!=imgIdx)
                {
                    imgIdx = value;
                    OnPropertyChanged(nameof (ImgIdx));
                }
            }
        }

        private string isbn;
        public string ISBN  
        {
            get
            {
                return isbn;
            }
            set
            {
                if(value!= isbn)
                {
                    isbn = value;
                    OnPropertyChanged(nameof(ISBN));
                }
            }
        }
        private List<string> imgsList { get; set; }

        private int imgsCount { get; set; }

        private int calcIdx = 0;
    }
}