<Window x:Class="GifAnimationViaWindowsFormsHost.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:Forms="clr-namespace:System.Windows.Forms;assembly=System.Windows.Forms"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
    <Grid.Background>
      <ImageBrush ImageSource="/GifAnimationViaWindowsFormsHost;component/Images/kbyte5x.png" Stretch="None" TileMode="FlipX" Viewport="0,0,233.304161611059,99.9874978333109" ViewportUnits="Absolute" />
    </Grid.Background>
    <WindowsFormsHost x:Name="_loadingHost" Width="128" Height="128" VerticalAlignment="Center" HorizontalAlignment="Center">
      <Forms:PictureBox x:Name="_loadingPictureBox" SizeMode="StretchImage" Width="128" Height="128"  />
    </WindowsFormsHost>
  </Grid>
</Window>

// код установки gif-а в PictureBox
public partial class MainWindow : Window
{
  public MainWindow()
  {
    InitializeComponent();
    // цепляем картинку
    _loadingPictureBox.Image = Properties.Resources.loader;

// см. файл проекта ниже
  }
}