Thank you Steven. I had not noticed that, which certainly explains why there was an error when trying to access the image page ... but I know that in 2015 when I created and uploaded the image, I got that link from the page and pasted it into the template code in a few profiles to which it applies, and I guarantee that I verified that it worked then.
I don't know if the numbers in the URL changed since then, nor do I understand why the error message would not say that the page does not exist instead of implying that it does exist and I am not logged in.