I would like to have a custom avatar but I don't know how. The message below is from the help page but it does not tell how to upload an avatar and use it.

"Our avatar system allows forum members to select an avatar (a small image) from our avatar collection, or to upload their own images to use as their avatars.

To select an image from our avatar collection, please click on the “Click to choose avatars” link located in My Profile page. Once you click on the link, a window titled "Click on the avatar you like”, will pop up and will show you a list of all the avatars to choose from.
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This site does not allow the use of custom avatars. You'll just have to use one of the ones available here.