You need to choose an animated GIF file with dimensions of 80x80 pixels or smaller in order for it to be displayed as an animated avatar. Anything larger will be automatically compressed by the GD Graphics Library, which currently does not offer well-functioning support for animated images.

If you have a piece of GIF animation software installed (e.g. the non-proprietary GIMP which works on most platforms, or CoffeeCup's GIF Animator for Windows), you can resize your animations of choice in it.

"Stephen Wolfram is the creator of Mathematica and is widely regarded as the most important innovator in scientific and technical computing today." - Stephen Wolfram