Okay first find the image that you want, save it to your computer and upload it to your own server. Then when your on the board, go to where it says "My Controls".
The "Menu" will be on your left, under Personal Profile, click Edit Signature.
Then where you uploaded your image copy the IMG code, and paste it in the box. Then under the code you can credit whoever made the baner for you, (everything will still work). Once you've done that, click "update my signature"
And everything should be set.
For an avatar, you do everything I've stated, but instead of clicking Edit Signature Settings from the Menu on the left, Click Edit Avatar Settings.
Then you can either upload something from your computer OR, (Find an avatar, save it to your computer, and upload it to your own server) Then get the URL and place it where it says "Enter a URL to an online avatar image" Then just click Update Avatar.
And everything should be set. Let me know if you need any more help.