Not the avatar – no.
But – if you are using the most up to date version you can attach a special badge to a user group so that each group could have a unique visual image. You could then turn off the use of avatars if you wanted to and more or less arrive at what you want I believe.

Ok will give that a go thanks for the help. just one quick question on the ranking, am I right in assuming the normal ranking badges are only applied once the user has reached the predetermined number of posts set? 

Certainly standard ranking can work that way. Badges and the use of ranking is optional of course. There is also a special ranking feature where you can nominate users with a special badge. These are all options.

I decided to go down your suggested route for the ranking which is working quite nicely so thanks for the advice.

Cheers

Glad it worked for you. And thanks for the confirmation.