I have run into this problem several times. The first time, I only uploaded the image and saved the post. Then once I confirmed that the image uploaded properly, I added my HTML text. Today, when trying to replicate this, I ran into trouble again. This time, I created my new post (title only). Separately, I uploaded my image in the media library and then attached it from there to my new post. After confirming the the image uploaded properly, then I added my HTML text. Hope that helps.
