In your User CP (nxt to FAQ & Rules) there is an option to 'Edit Signature'.
You can either type your 'doc' in there and it will display under every post you make or you can (as you say) scan it in. The image dimesions should be 400 x 120 pixels and no more than 25-30kb ish (less is better).
After you have added either 'words' or 'image' click on 'save changes'. Then your done.
For image hosting, most use Imageshack or Photobucket.
Edit - Right that's it.
You've all got to slow down. When a lady say's 'JUMP' do you all say 'How High'?:rotfl:
Go back to sea, all of you! That's an order.