private void button_Click(object sender, RoutedEventArgs e)
{
    ImageUrl1 = @"https://pixabay.com/static/uploads/photo/2016/02/09/13/45/rock-carvings-1189288_960_720.jpg";
    ImageUrl2 = @"https://pixabay.com/static/uploads/photo/2016/02/14/14/32/construction-1199586_960_720.jpg";
    ImageUrl3 = @"c:\test.jpg";
}

BackgroundWorker worker = new BackgroundWorker();
worker.DoWork += (s, e) =>
{
Uri uri = e.Argument as Uri;

using (WebClient webClient = new WebClient())
{
	webClient.Proxy = null; 
	webClient.CachePolicy = new RequestCachePolicy(RequestCacheLevel.Default);
	try
	{
		byte[] imageBytes = null;

		imageBytes = webClient.DownloadData(uri);

		if (imageBytes == null)
		{
			e.Result = null;
			return;
		} 
		MemoryStream imageStream = new MemoryStream(imageBytes);
		BitmapImage image = new BitmapImage();

		image.BeginInit();
		image.StreamSource = imageStream;
		image.CacheOption = BitmapCacheOption.OnLoad;
		image.EndInit();

		image.Freeze();
		imageStream.Close();

		e.Result = image;
	}
	catch (WebException ex)
	{
		e.Result = ex;
	}
}
};

worker.RunWorkerCompleted += (s, e) =>
{
	BitmapImage bitmapImage = e.Result as BitmapImage;
	if (bitmapImage != null)
	{
		myImage.Source = bitmapImage;
	}
	worker.Dispose();
};

worker.RunWorkerAsync(imageUri);