It doesn’t have to be much, even if it’s just a note embed in a demo page that you can’t edit, or a few components one could look at up close. When I go to your website I have to judge the entire product on 3 small images. Even a video would go a long way.

I also think you should show a tutorial before the sign-up page on your app. You may get organic traffic through the app store and potential customers have no idea if the app is for them before getting hit with a sign-up page.