Editing Posts Summary Page

What is the actual name of the php page that displays the post summary page? i.e. the page that shows a summary of posts with links to read more. I’d like to add a static image to that page just above the first post header, but have no clue which page to add it to. In my case the URL for that page would be https://www.website.com/blog

Any help would be greatly appreciated.

Thanks

John