Animated PNG files causing problems with some users isn't a problem with Twitter, I should think (and it seems it is independent of the file format, so whether it is GIF, PNG, or MPEG, it would be the same problem, I should think); it is the browser. The user should configure the browser to disable animations by default if that is what is wanted. (If the browser does not implement that, then the browser writer should fix it so that it does.)