<Window x:Class="WpfApp1.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"
          xmlns:local="clr-namespace:WpfApp1"
          mc:Ignorable="d"
          Title="MainWindow" Height="450" Width="800">
    
     <StackPanel HorizontalAlignment="Center" Orientation="Horizontal" VerticalAlignment="Center">
          <Button Content="Load Image" Command="{Binding MyModel.CommandClickLoadImage}" VerticalAlignment="Top" Margin="10"/>
         
          <Border BorderBrush="Gray" BorderThickness="1">
               <Image Source="{Binding MyModel.ImageSource, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Height="100" Width="100" VerticalAlignment="Top"/>
          </Border>
     </StackPanel>
    
</Window>

using Microsoft.Win32;
using System.Windows;
using System.Windows.Media.Imaging;
using System.Windows.Media;
using System.Windows.Input;
using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace WpfApp1
{
     public class MainWindowModel : NotifyPropertyChanged
     {
          private ImageSource _imageSource;
          public ImageSource ImageSource
          {
               get => _imageSource;
               set => SetProperty(ref _imageSource, value);
          }
          public ICommand CommandClickLoadImage => new ClickCommand(ClickLoadImage);

          public MainWindowModel()
          {
          }
          private void ClickLoadImage(object parameter)
          {
               OpenFileDialog op = new OpenFileDialog();
               op.Title = "Select a picture";
               op.Filter = "All supported graphics|*.jpg;*.jpeg;*.png|" +
               "JPEG (*.jpg;*.jpeg)|*.jpg;*.jpeg|" +
               "Portable Network Graphic (*.png)|*.png";
               if (op.ShowDialog() == true)
               {
                    ImageSource = new BitmapImage(new Uri(op.FileName));
               }
          }
     }

     public partial class MainWindow : Window
     {
          public MainWindowModel MyModel { get; }

          public MainWindow()
          {
               MyModel = new MainWindowModel();

               // for binding
               this.DataContext = this;
          }
     }

     ///// Base functionality /////
     public class NotifyPropertyChanged : INotifyPropertyChanged
     {
          public event PropertyChangedEventHandler PropertyChanged;
          protected void OnPropertyChanged([CallerMemberName] string name = null)
          {
               PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
          }
          protected virtual bool SetProperty<T>(ref T field, T value, [CallerMemberName] string PropertyName = null)
          {
               if (Equals(field, value))
                    return false;
               field = value;
               OnPropertyChanged(PropertyName);
               return true;
          }
     }

     class ClickCommand : ICommand
     {
          public event EventHandler CanExecuteChanged;
          private Action<object> _callback;
          public ClickCommand(Action<object> callback)
          {
               _callback = callback;
          }
          public virtual void Execute(object parameter) { _callback(parameter); }
          public virtual bool CanExecute(object parameter) { return true; }
     }

     ////////////
}