MyBB Community Forums

How do I add an avatar on the index and postbit of the last poster?

Postbit has avatar on each post, so I don't know what you're talking about.

As for index (and some other places):
https://github.com/TommM/flpavatar (download ZIP)

Then enable plugin's checkbox in ACP -> Configuration -> General Configuration and follow this:
https://github.com/TommM/flpavatar#on-the-forum-index