Thanks for the feedback! Let me try to address them one by one:
Users have noticed that sometimes they are sure they are posting a new topic to a certain category, but that topic goes in to a different topic (perhaps the one they were last in?).
If a user opens a "new post" window in one category, that post window is tied to that category. If you switch categories and still use the same posting window, the new post will appear in the category you were in prior.
When users load their own image/avatars from local resources, they stay in the system while they are logged in - but the next day those images are gone/broken.
Heroku's file system is "ethereal", meaning that every time you restart your Heroku, all of your stored data (that is, things like any uploaded avatars) are lost. It is an inherent limitation of Heroku, and can be theoretically worked around by using a third party image hosting service. NodeBB doesn't support this yet, but you can take advantage of Gravatars, which brings me to...
One user put in his email address and was surprised to see an unexpected old picture of himself, that he had no idea where it came from. He did not think it was from his known social network ids. This bothered him. Maybe a clear understanding of where NodeBB can extract pictures based on email would be helpful.
NodeBB uses "gravatar"s by default. A gravatar is an image you register once (on gravatar.com), and it can be used in all sites that support them.