Simply doesn't work as expected

I installed and activated the plugin, and could add a picture to a category. But on the front category page the picture did not show up. I found a notice that I’d have to modify some php files but I’m not a developer, I’m a simple user. And there is no category.php file in my twenty fifteen theme folder which I could have copied over in the child theme folder before editing it.

Please make clear instructions: Which file at which location in the folder tree to modify on which line, if it is already not possible to install the plugin without requiring the user fiddling around with code.