You're welcome, but just as an update to my previous response, since then, I have made PageImages available to be individually enabled in Special:ManageWiki/extensions (No longer requires you to enable popups also)
[...]PageImages available to be individually enabled[...]
Yeah I've noticed that when I tried modifying the permissions. First I thought there was an inconsistency between the mobile and desktop version, because "Page Images" wouldn't show up on desktop, but this problem has fixed itself. Thanks for untangling those two permissions.
I've also just noticed the "Popups" extension can now be activated correctly.
It was locked yesterday, even though "Page Images" and "Text Extracts" was already activated for our wiki.
In other words, everything is working perfectly. :)
Thanks again for your help!