It's to big and the forum must be resizing it and making a mess of it with it being animated.
When you upload an avatar is says
Quote:
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Note: The maximum size of your custom image is 100 by 100 pixels or 39.1 KB (whichever is smaller).
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Yours is 282KB and 480 x 320 pixels.
This website can resize it for you
Online GIF Resizer - Resize Animated GIF Image