To change the banner, see the IPB documentation on how to change your board logo:
http://forum.forumer.com/showthread.php?t=6064
You can upload custom avatars by going to "My Controls" (top right corner) -> Avatar settings and click the browse button. Choose the image you want and press "upload".
Before you can do this you need to have it allowed for your usergroup:
ACP -> Manange Usergroups -> Edit (Admin)
Make sure that the box called something similar to "Allow avatar uploads"
is checked
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Moridin