Although if I copy/paste the image URL into the browser address bar and access the image directly (with a HTTPS URL) it displays.
It's been my experience that this behavior is usually caused by "Hotlink Protection" in your web hosts cPanel (or the settings therein).
Bingo! Thanks, Jim - it was indeed Hotlink Protection. I had added this functionality in .htaccess while applying a security-related contribution. Thanks for your swift help, Jim (and Phil).