Hi metanite, I apologize for bothering you but what website did you find your avatar; I have been looking for a HGSS TTar or Suicune but all of them that I have loaded haven't appeared as the full image such as yours. Thanks and I hope to hear from you soon =]
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The reason that MetaNite's avatar is larger than ones you have tried to upload is because he is a badged member, and as such is allowed to have a larger avatar and other special privelages. If you are wondering how to earn a badge, then be sure to read the "badge and trophy faq" in the FAQ section of the site.
Welcome to Smogon, enjoy your stay! ;)