You should be able to change your profile picture on the mobile app and be able to do all the account customization there as well. It would make things easier so that we wouldn't have to hunt down a computer if we don't have one.
To be honest, I can't seem to find a way to change my profile picture FROM my computer at all. This is quite irritating, and from what I've heard you can only change it through the Mixer Creative app. I'm in I.T. but don't use a portable device and I am finding this quite a strange feature to be hidden... (if it exists at all).