Missing images in FlowDocument saved as XPS document

The eventual solution was the same as you came to, which is to put the document in a viewer and briefly show it on screen. Below is the helper method that I wrote to do this for me.

private static string ForceRenderFlowDocumentXaml = 
@"<Window xmlns=""http://schemas.microsoft.com/netfx/2007/xaml/presentation""
          xmlns:x=""http://schemas.microsoft.com/winfx/2006/xaml"">
       <FlowDocumentScrollViewer Name=""viewer""/>
  </Window>";

public static void ForceRenderFlowDocument(FlowDocument document)
{
    using (var reader = new XmlTextReader(new StringReader(ForceRenderFlowDocumentXaml)))
    {
        Window window = XamlReader.Load(reader) as Window;
        FlowDocumentScrollViewer viewer = LogicalTreeHelper.FindLogicalNode(window, "viewer") as FlowDocumentScrollViewer;
        viewer.Document = document;
        // Show the window way off-screen
        window.WindowStartupLocation = WindowStartupLocation.Manual;
        window.Top = Int32.MaxValue;
        window.Left = Int32.MaxValue;
        window.ShowInTaskbar = false;
        window.Show();
        // Ensure that dispatcher has done the layout and render passes
        Dispatcher.CurrentDispatcher.Invoke(DispatcherPriority.Loaded, new Action(() => {}));
        viewer.Document = null;
        window.Close();
    }
}

Edit: I just added window.ShowInTaskbar = false to the method as if you were quick you could see the window appear in the taskbar.

The user will never "see" the window as it is positioned way off-screen at Int32.MaxValue - a trick that was common back in the day with early multimedia authoring (e.g. Macromedia/Adobe Director).

For people searching and finding this question, I can tell you that there is no other way to force the document to render.

HTH,