If you go into "Edit your Profile" you can add an avatar (under your user name) or a pic/message etc in the signature part of the profile But be careful of the sizing rules - Admin will be on to you
Best way of getting pics up is to open a photobucket account where you can also resize and edit pics. By clicking onto the IMG code under uploaded pics you can then copy into the reply/post you are making.
Any probs just shout!
as colin said, all from your profile. Once you have a picture which is no bigger thann 500 pixels wide X 100 pixels high. Upload that picture to a photobucket account that you ahve set up. Then use the IMG code it supplies once you have uploaded the picture and paste that in your signature bit in the profile