How to convert authenticated html web page to image using ruby on rails application or Javascript

I have a 'design' web page which have some design templates frame with some theme images and colors. and i want to convert this web page to an image. I tried to use imgKit, wkhtml2image gem but it is only displaying Login page as image, It is not showing the page which we require. I tried to go with "html2canvas" as well but it only displays layout. It does not display the page content. Please see the code:

def design_update
  respond_to do |f|
    f.json do
      just_promo_params = promotion_params
      theme_params = just_promo_params.delete(:theme_attributes)

      if ThemeEngine::UpdatesThemeWithHistory.update_with_promotion(
        promotion,
        just_promo_params,
        theme_params,
        current_user)

        ASNWithControllerContext.instrument 'promotion.saved_design',
        context: self,
        promotion: promotion

        render status: 200, json: {errors: []}
      else
        render status: 200, json: {errors: [promotion.errors.full_messages]}
      end
    end
  end
end

I have to convert this json content into image. Any ideas how to do this?