You can upload an avatar by going to your user page (click on username) and selecting "Edit My Profile". You must have over 5,000 points in order to upload a profile avatar. Personal pictures are not allowed.
Source: KidzTalk FAQ
You need at least 5,000 points if you want to get a profile picture. I waited for so long until I got to change my profile picture, but I bet you'll get 5,000 points faster than I did.
~ Your friend, Wraya <3