Great post!! upvoted! 🔧 This error happens also sometimes when you use markdown specifically for code, for example there is markdown to show code like this one for the strong tag:
<strong>text here</strong>
But I encountered this "An image in this post did not save properly" error when I tried to show code for the img tag instead of strong. It appears that the img tag creates issues when being displayed bia code markdown in a post.
I just wrote a post https://steemit.com/spanish/@felipesuarez/alinear-imagenes-izquierda-derecha where I wanted to show code on the post using the markdown to display the code. But I had to omit that and change the code part for an image showing the code instead to avoid seeing the "An image in this post did not save properly" error which does not allow to publish the post.

Seeing this error creates a lot of confusion, specially since you want to publish your post fast, so take into account 👍