In the mean time I did the obvious fix: renaming my loading image to WordPress Ball Spin.
Edit:
I believe my issue is that I can’t edit the plugin .php file – at least the website doesn’t respond to the changes I make (even random stuff like adding typos to “Loadinggg Please wait”) in WordPress’s plugin editor.
I checked the .php via Filezilla and it *is* modified.
Edit2:
Nevermind, I got it to work now. I surely didn’t pay attention to the fact that it only reacts to changes made under Settings -> Advanced Ajax Page Loader.