How do I upload a custom avatar?

Just go to your Account Settings page, where you would normally select an avatar. Once there, you can pick a normal WoW icon like before—or you can select "Custom Icon". Once you do, you'll be presented with a drop-down menu that you can use to either select a custom icon you've previously uploaded, or upload a new one.

Once you've uploaded a few icons, you can manage them using the Image Uploads section at the bottom—this lets you sort your uploads, and even give them humorous titles so you can remember them. :)