You can now upload what is called an Avatar – a personalised image that appears beneath your username on all your posts in these forums. Do so by editing your profile, the Avatar section is at the bottom of the profile page. Please note restrictions apply to the maximum size of the image (max width and height 80 pixels, max file size of 100KB). Choose a favourite image, photo, icon etc to create your image, and resize it (you can resize an image by using a photo editing program or a free online service such as Create Avatar).

http://www.heysentrail.asn.au/forums/faq.php

How do I show an image below my username?
There may be two images below a username when viewing posts. The first is an image associated with your rank; generally these take the form of stars or blocks indicating how many posts you have made or your status on the forums. Below this may be a larger image known as an avatar; this is generally unique or personal to each user.