the avatar is done in your preferences page once in there you will see the avatar option which will bring up the ones you have uploaded and will allow you to delete them and add new ones. just simply pick which one you want to use and hit the update button at the bottom and viola.

the sig is done in your profile page which is also found in your preferneces page just uplaod the sig pic and then get the adress for it and make sure to add the img and img/ in these [] boxes for each one and add it in your sig boxes and then hit the update button at the bottom and once again viola.