From PG Rules:
Avatars Options:
"Members are allowed to use either animated or static avatars with the max dimension of 100x100 pixel and max size of 50KB. Your avatar must not include any image or text which could be deemed as obscene, vulgar, sexually-orientated, hateful, threatening, illegal or otherwise volatile of any forum decency standards."
The image you are referring to is:
Dimensions: 113x119
Weight: 79KB
So bascially, its exceeding BOTH requirements set in PG Rules.
If you know a little bit of Photoshop (or if you have it in your PC), just search for "Reducing file size of a Gif" and you'll greeted with a lot of tutorials. You will only have to "Posterize" and "Resize" this gif and you'll be fine.