Currently there is no standard custom avatar system in place. There were plans for a remote-linked (url) avatar system but they've been shelved until after we upgrade to the new board software. Other than a couple of members who were used to test the syste, your url avatar is just your board one.

To see the url avatars, go to UserCP/Edit Options, and check the Display Url Avatar box. If you don't want to see both url and board avatars, you should then disable Show Avatars in the Visible Post Elements box. Then go the thread Craterus mentions in post 2, and look at Uesugi Kenshin and TosaInu's avatars.