At the top of the forum page there are a number of links you can click that relate to the forum. Some of these are - FAQ, Search, Mail and Profile. Click on "Profile" and it will take you to your profile page and there you will see such things as - Username, current password etc.
Further down you will see -
For signature, there is a text box for you to write in. Here you can write what you want and even place pictures but to place pictures you must place [img]the%20url%20of%20your%20pic[/img]
The image has to be on an internet site (any site) so it can be linked to.
At the bottom of your profile page there is a section called - Avatar control panel.
Here you can chose from three categories -
Upload Avatar from your machine:
Upload Avatar from a URL:
Enter the URL of the location containing the Avatar image, it will be copied to this site.
Link to off-site Avatar:
Having a pic on your computer and thus picking the first option (Upload Avatar from your machine: ) is probably easiest.
Hope this explains it