I would like to use a custom avatar. I have cropped it and reduced it to 100x100 pixels, but I do not know where to post it.

You'll need to get a picture hosting account, somewhere like photobucket, and link to the picture there.

Then when you have a site with the avatar, post the URL on the bottom of the first page of "Select a Different Avatar" on your Profile page here and it will go and fetch that avatar.

Artemis