There's unfortunately some people who encounter problems with changing their avatars and I'm not exactly sure why that happens. Some things that work at times is either try to clear the cache and cookies of your web browser before making the change or try to do it in different browser. It doesn't always work though, so you might have to wait and try again at a later date if it doesn't. Sorry about that.
With that said, the work you posted look pretty nice! It will be cool to see how it evolves as you go through the course 
