为什么这个绑定不能在MVVM中工作?

我正在从WPF视图设置到ViewModel对象的绑定，但是绑定似乎没有被触发。我在MVVM中没有做太多的工作，所以我想我应该问一下，当页面加载时，绑定到图像的源属性是否有原因。

页面的XAML:

<Page x:Class="DallasPrintManager.PrintPage"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
  xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
  xmlns:viewmodel="clr-namespace:DallasPrintManager.ViewModel"
  mc:Ignorable="d" 
  d:DesignHeight="300" d:DesignWidth="900"
  DataContext="{Binding Main, Source={StaticResource PrintPage}}">
<Grid >
    <Grid.ColumnDefinitions>
        <ColumnDefinition  />
        <ColumnDefinition Width="200" />
    </Grid.ColumnDefinitions>
    <ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
        <Image Source="{Binding ImageDisplay}" />
    </ScrollViewer>
</Grid>
</Page>

这里是ViewModel:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
using System.Windows.Media.Imaging;
using System.Drawing;
using System.Windows.Input;
using DallasPrintManager.Commands;
using System.Net;
using System.IO;
using System.Windows;
public class PrintPageViewModel : INotifyPropertyChanged
{
    private BitmapImage _imageDisplay;
    public PrintPageViewModel()
    {
        ImageDisplay = getImage();
    }
    private BitmapImage getImage()
    {
        try
        {
            WebClient wc = new WebClient();
            byte[] imageData = wc.DownloadData("http://localhost/TestImage.tif");
            MemoryStream ms = new MemoryStream();
            ms.Write(imageData, 0, imageData.Length);
            System.Windows.Media.Imaging.BitmapImage wpfImg = new System.Windows.Media.Imaging.BitmapImage();
            wpfImg.BeginInit();
            wpfImg.StreamSource = ms;
            wpfImg.EndInit();
            return wpfImg;
            //return (Bitmap)Bitmap.FromStream(ms);
        }
        catch (WebException e)
        {
            MessageBox.Show("Error fetching document:'n'n" + e.Message);
            return null;
        }
        catch (Exception e)
        {
            if (e.Source == "System.Drawing")
                MessageBox.Show("Error reading document.");
            Console.Out.Write(e);
            return null;
        }
    }
    public BitmapImage ImageDisplay
    {
        get
        {
            return _imageDisplay;
        }
        set
        {
            if (value != _imageDisplay)
            {
                _imageDisplay = value;
                OnPropertyChanged("ImageDisplay");
            }
        }
    }
    public event PropertyChangedEventHandler PropertyChanged;
    protected void OnPropertyChanged(string propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this,
                new PropertyChangedEventArgs(propertyName));
        }
    }
}

视图模型在app.xaml中实例化，并被打印页面绑定。

<Page x:Class="DallasPrintManager.PrintPage"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
  xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
  xmlns:viewmodel="clr-namespace:DallasPrintManager.ViewModel"
  xmlns:diag="clr-namespace:System.Diagnostics;assembly=WindowsBase"
  [...]
  DataContext="{Binding Main, Source={StaticResource PrintPage}}">

<Image Source="{Binding ImageDisplay, diag:PresentationTraceSources.TraceLevel=High}" />

public class DebugDummyConverter : IValueConverter
    {
            public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
            {
                    Debugger.Break();
                    return value;
            }
            public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
            {
                    Debugger.Break();
                    return value;
            }
    }