<Window>
...
  <Window.Resources>
    <wpf1:CMyMultiConverter x:Key="MyMultiConverter"/>
    <wpf1:CMyConverter x:Key="MyConverter"/>
  </Window.Resources>

  <Grid>
    <StackPanel Orientation="Vertical">
    

      <!--Работает-->
      <Image Source="/Image1.png" Height="50" Width="50" Stretch="Fill"/>
			
      <!--Работает-->
      <Image Height="50" Width="50" Stretch="Fill">
        <Image.Source>/Image1.png</Image.Source>
      </Image>
			
      <!--Работает-->
      <Image Height="50" Width="50" Stretch="Fill" Source="{Binding Converter={StaticResource MyConverter}}" />

      <!--Не работает ??? -->
      <Image Height="50" Width="50" Stretch="Fill">
        <Image.Source>
          <MultiBinding Converter="{StaticResource MyMultiConverter}" />
        </Image.Source>
      </Image>

    </StackPanel>
  </Grid>
</Window>

public class CMyConverter : IValueConverter
{
  public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
  {			
    return @"/Image1.png";
  }
  ...
}

public class CMyMultiConverter : IMultiValueConverter
{
  public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
  {
    return @"/Image1.png";
  }
  ...
}