//This is a custom resource handler that fails on the background 
//image Url on purpose so that it won't be rendered
private class CustomResourceHandler : EO.WebBrowser.ResourceHandler
{
    public override bool Match(Request request)
    {
        return request.Url == your_background_image_url;
    }

    public override void ProcessRequest(Request request, Response response)
    {
        //Fails on purpose so that it won't be rendered. Alternatively, you can
        //render a replace image here
        throw new NotImplementedException();
    }
}

//You must use a HtmlToPdfSession object since we need
//the RunWebViewCallback method offered by this class
using (HtmlToPdfSession session = HtmlToPdfSession.Create())
{
    //Register the CustomResourceHandler with the underlying
    //WebView object that is responsible to render the page
    CustomResourceHandler handler = new CustomResourceHandler();
    session.RunWebViewCallback((webView, arg) =>
        {
            webView.RegisterResourceHandler(handler);
            return null;
        }, null);

    //Load the Url into the WebView 
    session.LoadUrl(html_file_name);

    //Render it as PDF
    session.RenderAsPDF(pdf_file_name);

    //Unregister the custom handler
    session.RunWebViewCallback((webView, arg) =>
    {
        webView.UnregisterResourceHandler(handler);
        return null;
    }, null);
}