In order to have a custom avatar you have to be a member of TCS for a certain amount of time and have the level of 'Super cat'.
I speak under correction, but I remember reading this info in a thread in this forum, just this week.
Edited to add:Super Cat
- 700 posts
- 120 days in membership
- 40 stored PMs
- 39 additional avatars to choose from
- Ability to upload custom avatar
- 80 Kb in uploaded attachments
- Posting access to IMO, if over 18.