How do I remind users to pick an avatar? I use the Avatar Selection Module to provide a list avatars my users can choose from. I don't want to make choosing an avatar required, but I'd like to have a persistent reminder for users who have not picked an avatar for themselves.

How can I identify users who have NOT chosen an avatar and use Context (or something else) to conditionally display a message in a block like, "Hey you haven't chosen an avatar yet. GO HERE (User Form) to choose one."?
Profile Complete Percent helps to guilt users into filling their profile details. This isn't avatar specific, but you can configure the PCP block to only use a single field for its percentage calculation.