Ok, here's what you want size-wise for an avatar:
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(dimensions width 75 x height 90 - you'll put those into the space necessary when you upload the avatar to this site).
For the avatar, you'll go into your control panel - and where you put in the picture, you'll have to use a different code (after you upload this one) - use the DIRECT link code for that - put in the dimensions I gave you here - and voila, you'll have a signature picture.
(If you aren't sure how to save this picture - right click on it and save it to your computer, then upload it to Photobucket and it'll be this size).