NationBuilder donation pages — along with all of the pages of your control panel — are secure and use state of the art industry encryption technology. You will know you are on a secure donation page when you see "https" at the start of the URL.
If you see an error on the security of your custom donation page, it is either because you are using photos hosted in insecure locations, or template code that is not secure, such as external "share" widgets. To secure your custom images, host them in the Files section of your donation page. And if you have site-wide widgets that are not secure, you can suppress them from your donation page by using this code in your template (exclude quotation marks):
{% if request.is_ssl? %}
{% else %}
code goes here
{% endif %}