Avatars must be sized to be no more than 180x180. Use any image editing software & adjust the size. It is best to save your avatar in the .gif format. To upload your avatar, go to your account and use the upload picture feature to upload your avatar.
If you want to change your avatar, go to your account and select the "delete picture" checkbox and click submit. Now you can
browse to your new avatar image and upload your new picture. You cannot delete current picture and upload new picture all in the same step.