Customizing post on Seventeen child

Hello everyone,

I made a child theme of seventeen and I created a full-width-post template.
I was wondering how I could customize the post to have a two columns content and a post-footer-like where in those two columns post an image is mandatory in left column and contents in right column, while in post-footer-like a gallery is mandatory like a post theme? And how I could put in the post editor something like indications to say “insert an image here”, “insert content here”, “insert gallery here”?

Thanks in advance!
Cheers,
Luigi