Sometimes, your customers will be placing a reservation online, their browser may give them a security warning.  Currently, the URLs of websites that use HTTPS are labeled “Secure” by most browsers; when websites that do not use HTTPS are loaded, most modern browsers warn their users that the site "Not Secure." 

You'll need to ensure each image is hosted securely, meaning the beginning of the link starts with "HTTPS://". All images uploaded directly to BRM are securely hosted, while any images LINKED into BRM must be on a secure host (starting with HTTPS:) in order to avoid the security warnings.