I came here looking for help trying add some images to the space next to the story itself. I have managed to get the index.md in the content/posts/post-name directory working as it were a content/posts/post-name.md.
I moved your question to a new topic as the one you were replying to is 2 years old now.
The reason you cannot use page resources as the sidebar image is because the theme you are using constructs the img src urls with the assumption that your images are in static/images/. You can see that here and here.
You could either ask the theme maintainer to add code to support your use case, or you can overwrite the two files I linked to above yourself.