gistfile1.cs

Example XAML:

<ListBox.ItemTemplate>
    <DataTemplate>
        <StackPanel>
            <ProgressBar x:Name="PART_ProgressBar" />
            <Image>
                <Image.Source>
                    <BitmapImage behaviors:ImageDownloadProgress.ProgressBar="{Binding ElementName=PART_ProgressBar}" UriSource="{Binding Image}" />
                </Image.Source>
            </Image>
        </StackPanel>
    </DataTemplate>
</ListBox.ItemTemplate>

C#:

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media.Imaging;

namespace Behaviors
{
    public class ImageDownloadProgress
    {
        public static readonly DependencyProperty ProgressBarProperty =
            DependencyProperty.RegisterAttached("ProgressBar",
                typeof(ProgressBar), typeof(BitmapImage),
                new PropertyMetadata(null, OnProgressBarChanged));

        public static ProgressBar GetProgressBar(BitmapImage selector)
        {
            return (ProgressBar)selector.GetValue(ProgressBarProperty);
        }

        public static void SetProgressBar(BitmapImage selector, ProgressBar value)
        {
            selector.SetValue(ProgressBarProperty, value);
        }

        private static void OnProgressBarChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var image = d as BitmapImage;
            if (image == null)
            {
                throw new ArgumentException(
                    "You must set the Command attached property on an element that derives from BitmapImage."
                );
            }

            var oldCommand = e.OldValue as ProgressBar;
            if (oldCommand != null)
            {
                image.DownloadProgress -= DownloadProgress;
            }

            var newCommand = e.NewValue as ProgressBar;
            if (newCommand != null)
            {
                image.DownloadProgress += DownloadProgress;
            }
        }

        private static void DownloadProgress(object sender, DownloadProgressEventArgs e)
        {
            var selector = sender as BitmapImage;
            var progressBar = GetProgressBar(selector);
                        
            progressBar.IsIndeterminate = e.Progress != 100;
        }
    }
}