Oh don't worry too much about this. Usually, people will use different avatars anyway. If someone really impersonates you, like same MSN and same avatar, just report the person to the local admin and tell him your IP address. He'll see both won't match and take action if it's against his rules.
There are even people who has different nicks over each forums. On Cemetech I am xlibman, on Ticalc.org I am Illusiat (altough I have my real name showing up instead, which is Kevin Ouellet), but I hardly post on other forums anymore in general.