The ability to swap images but not text seems arbitrary.

You could imagine a system more like the notification tray on iOS/Android where at any time a notification can appear, be edited, timeout, or be deleted.

Your email inbox could be like that. The email saying "Your parcel has been dispatched" could be edited to say "Your parcel has been delivered".

When you refund something you've bought, the original purchase receipt could be crossed out or hidden. When you get invited to a wedding but then the wedding is cancelled, the original invite could be deleted, etc.

It's counter to the principle of what e-mail is. It's supposed to be static. Just because you can doesn't mean you should.

specifically to prevent this kind of tracking

Bet they send a separate mail when you paid though, in which case updating the picture is not much more than a means for them to hide errors.

I subscribed to the daily headlines from a newspaper, they delivered them as a remote picture in the mail. Only it was always the same remote picture each day, just updated. So if you didn't open the mail each day too bad: you snooze you loose, those past headlines are gone.