Is this done in the loop-blog.php?
It depends on when your thumbnail should appear. The loop-blog.php is used when showing the “posts page” (multiple posts) while the loop-single.php is used for single pages.
I’m struggling with the sample code.
What sample code?
On both loop-blog.php and loop-single.php the header text is added immediately after the opening <article> tag. You might want to add your image there.