The [img] BBCode is an internal BBCode for phpBB. I would not want to overwrite that, in case it breaks something.
The way phpBB works, in the worst case I would have to reparse every post with the tag ever made.
Also with every update i would have to recheck if it is still working.
Not that this is a big problem, I already did that twice with the youtube tag, but it is just an insane effort compared to the benefit
(With the youtube tag I had no choice, as it was breaking the other video codes. Now it is part of an extension, so I got that out of my hair
)
So I would prefer to stay with the official code of phpBB and when they decide to change the behaviour of something, they also provide us with the migration scripts.
I am also installing official extensions, so if something comes up in that direction, I will consider it, of course.
I hope that expalins some stuff.
Regards
Wolf