I guess that some people just find animated images annoying, especially in forums, where people come and go to read or post. That's why some forums don't allow animated avatars and signatures. Unfortunately, we can't avoid that, especially on a large forums such as UM.
Some people find it funny, some people find it annoying, some people can't be bothered, it's just how it is.
But it all comes down to who owns those avatar and signatures. Mostly, they describe who or what the owner is like.