The texture must be a 256 pixels by 256 pixels in 32 bits colors in .png format.
The easiest way to create your own texture is to download the default texture here and then modify it with your paint tool.
Now your texture is ready, you have to put it on the server and attach it to your profile:
- Click on "User Control Panel" on the top bar
- Select the "Profile" Tab
- Click on "Edit avatar"
- Select the texture you made in the "Upload from your machine:" field
- Click on "Submit" button
After that, the avatar is immediately available on posts you wrote on the forum, in your web profile and also in the game.