If the intention is to host the images on your own server instead of the cdn, then it's not as simple as a single file edit.
It's...
a language string update, to handle the display of a small number of items in the ACP (that's what the issue was in that linked topic)
a modification of core ips JS which contains the hard-coded cdn URL (JS which isn't stored in a local file, but in a database table)
a modification (via plugin/app hook, ideally) of a function contained in IPS\Output