No, I mean to send a small suggestion every 500th post. Not really too unpleasant.
The message would be like...
"Hello, we greatly appreciate your participation in the forum. We would enjoy for you to continue using our services. However, we've noticed you have not yet made an avatar for yourself. Though we honestly don't care, it makes the forum more enjoyable for yourself and others. It allows you to develop a reputation on the board. When you don't have an avatar, some people may not take the time to read your name, and it gives no credit to you. Having an avatar makes you easily identified.
If you don't know how to make an avatar, see here<---(linked). If you don't want, or feel the need for an avatar, that's fine. It is not required, and does nothing more than make it easier to make friends.
If you don't want to receive this message again, send a PM to ...... saying, 'I would like to stop receiving messages about not having an avatar.'"