Pro Sites: Secure image for PayPal

In Pro Sites settings for the pay pal (optional) header image it states the following:

“PayPal recommends that you provide an image that is stored on a secure (https) server. If you do not specify an image, the business name is displayed.”

I’ve gotten the SSL certificate on the server. What do I need to do to securely host an image? Is it just another directory with special permissions?