Downloads of files like images may be banned if they use HTTP connections – even if they are available from an HTTPS website.
Google Chrome will soon restrict certain files, like PDFs or executables, from being downloaded via an HTTP connection, if they are loaded on HTTPS webpages.
HTTPS indicates that a website has an encrypted connection. When connecting to an HTTP website, browsers merely look up the IP address and send data over to it in cleartext. When using an HTTPS website, on the other hand, the browser checks that it has a legitimate SSL certificate before sending data in encrypted form – preventing man-in-the-middle (MiTM) attacks and more.