Yugi - sure, it's actually really easy.
1) At the top of the page, click on PROFILE
2) Then click on EDIT OPTIONS
3) Scroll all the way down and click on CHANGE AVATAR (it will say thanks for chaning your profile, but don't worry - it's moving you to the page)
4) Once that page loads, also scroll down to the bottom. You
MUST CHECK YES:
5) Then, if you have a avatar stored on your computer, you can click browse, select where the file is (100x100 is max size) or you can put one up from the internet.
6) Click SUBMIT MODIFICATIONS and you're all set.
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