Click the link 'My GTPlanet' in the top left hand corner of any GTP page, and then using the links of the left of the page (under 'Control Panel'/'Settings & Options'), click Edit Avatar... you should be able to change your avatar... make sure it's within the size limits (both pixels and memory)... if your pic is too big, try resaving it in Photoshop (or similar) using the 'Save for web...' option, as this cuts down the size of the file with minimal lose of quality to the image.
edit: since you only just made your 100th post, there may be a slight delay before you can access this option. That may be the problem... if it is, you may need to wait for a few hours or until tomorrow before you can change your avatar...
