Firstly you need an image host account, for example Photobucket (which is free) to upload your signature to.

Signature
Go into "my controls" then select "edit signature" from the list on the left and in the edit box there is a button with a picture on it, click that button then paste the url from the image host in the pop up box.
Save your settings by clicking update signature and you're done.

to center your signature click the button in the middle of the 6 buttons on the top right hand side (the one with the lines in the middle).


Avatar
For the avatar just paste the image url into the middle box or upload one from your computer by clicking the browse button (the one below the url box), when thats done just click update avatar.

It's better to use an image host for your avatar as attaching them slows the loading of the pages


thats it!
simple eh?