Yeah, that sounds important. If we can do it without cluttering the UI too much, here's my thinking:

The main "post as" selector lists your alternatives, and if you change who you're posting as, it assumes you want the default icon, etc. That way, we don't have to do much for what I suspect is a common case. But when you change, you also get the option to fetch icons/moods/etc for the new identity.

Preloading icons and moods for multiple identities would suck, resource-wise. This adds an extra step, but it doesn't need to be too intrusive, and it's still easier than switching to another account entirely.