My theme contains a parallax header image. How can I change the *URL* of the header image so that it uses `HTTPS`? I want to do it manually.

Currently the URL is:

    http://example.com/themes/abc/header.jpg

I want to change it to:

    https://example.com/themes/abc/header.jpg

Plugins are not helping me on this specific file (they are converting others). Chrome tells me the header is being loaded by `jquery.js`. I have no idea how jQuery is doing this.

I believe editing `functions.php` can help, but how can I do it?