You spent more than 15 minutes changing wallpaper in Plasma? I smell bs because it’s really simple.
Right-click on desktop:
Pick an image from the list, or add your own images (“Add…”):
After you click “Add…”:
The last windows I used was 10, and I remember the process being very similar.
Sorry this happened.
Use it as an opportunity to learn how to better store and edit your code (e.g. a VCS and a smart-ish editor). For me, a simple Ctrl-Z would be enough to get my code back.