by Rhonda Waldrop.
I can tell you what I have discovered the times something similar happened on our site.
When I first started working with MOODLE, we had this problem alot. The instructors were actually (gasp) copying and pasting images and content from word documents. Once I got them out of that challenge...then our site changed from http to https and that caused similar challenges. So, if you know a site is having a problem. I would double check and confirm that the image is actually linked to the image that is in the folder. Sounds weird and to this day, most swear they did not copy and paste, but the evidence indicates other wise So, I will chalk it up to a fluke.