It hasn't been working for me, I've been trying to change mine for a few days now. For some reason it just doesn't upload a new one - I select the file and press upload (or whatever it says) and nothing happens.
But anyway, yeah. It has to be 50x50 (you can resize it on your own), save it, then go to Manage My NS and it says click to change icon or something right there. But if it doesn't work it's not just you having problems!