[MarkupExtensionReturnType(typeof(System.Windows.Media.Imaging.BitmapSource))]
[ContentProperty("Key")]
public class ImageSource : MarkupExtension
{
    public ImageSource()
        : base()
    {
    }
    public ImageSource(string key)
        : base()
    {
        Key = key;
    }

    public string Key
    {
        get;
        set;
    }

    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        BitmapImage bi = new BitmapImage();
        bi.BeginInit();
        bi.UriSource = new Uri(string.Format(@"{0}/{1}", Environment.CurrentDirectory, Key));
        bi.EndInit();
        bi.Freeze();

        return bi;
    }
}