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. It is up to the board administrator to enable avatars and they have a choice over the way in which avatars can be made available. If you are unable to use avatars then this is the decision of the board admin and you should ask them their reasons (we're sure they'll be good!)