So I looked what happened when I ONLY change the bg image and the same thing happened again, so the image itself was the issue. I tried resizing it to the same format and resolution as your background and while the stretching stopped I realizied it's not positioned right, repeating itself on the right side of the profile (it's just not apparaent with the pixel art bg). Though if I change the size it's doing the weird stretching thing again.

Now I just don't know how I get the image to be positioned correctly. It's set to center but it's clearly leaning to the left.

EDIT: Just a note that I tricked a bit with the image I used so it's not visible that there's an edge of a repeating image on the right side. It's not actually fixed