This members-only tutorial provides the steps to set up images for categories and display them as backgrounds for a Section below the header on the corresponding category archives in Oxygen.
Backend:
Frontend:
Note: This works with not only categories but also tags and custom taxonomies.
Step 1
Install and activate WP Term Images plugin.
Edit your categories and select/upload your desired images.
Step 2
Install and activate Code Snippets plugin.
Add a new Snippet named say, "Retrieve WP Term Image URL" having the following code: