If you know the email addresses, user name and password of each member, you can log into the forum as if you were them - and restore their avatars by clicking on their personal settings. I did it for five members, out of the goodness of my heart, then gave up
Of course, to do this, you'd need some way - separate to the forum structure - to capture their passwords when they join. For example, by using a member enrolment form at your site that gives you other useful data eg. their zipcode, personal interests, etc. But then, some folk will change their passwords once they're on the forum itself... and you're back where you started.