You can host ONTRAPORT forms on a secure landing page. For legacy Smart Forms and ONTRAforms we use code that "inherits" the parent page's settings.
If you use any external images make sure they are hosted on a service that is also secure. If you use an image from a non-secure server it will cause an error in the client's browser. To be safe, you can upload your images into the ONTRAPORT image manager and include them in your form from there.