You are right that the change would work. However, it would then be very different from the native IMG code. Maybe that doesn't really matter.
I was thinking of it as a modification of the existing code. I could just make up a new name entirely and avoid any confusion.
I think you won't have that problem. See, by doing it that way - you're keeping with the standards set by the URL bbCode. This makes more sense to a lot of people since you would be treating them both the same way.
If you really wanted to modify the way the IMG bbCode is handled, you can try using a plugin... But it would be overly complicated and unnecessary.