I have seen this in a post of mine today as well as another's. It seems like when using markup and embedding an image (via URL), that even if the link to the image is valid, once the post is created, it becomes a deadlink. I'm not sure if the image is changed to a cached version of the image which is screwing up, or if the processing/altering of the link changes it to something faulty.
Just a heads-up.
Addendum: Referenced post: https://steemit.com/photography/@dv8silencersteem/yesterday-was-a-beautiful-day-in-madison-wisconsin
Addendum: See my comment below on how I fixed it.