Yes, the domain webphoto was for the example. so it doesn't work.
But the problem exist with a real domaine.
The forum is based on intranet access, like
http://intranet/forum (it's an example).
The URL I had entered was like :
http://webphoto/photo/photo1.jpg (URL valid).
This URL only works on intranet mode, and when I put this type of URL for the avatar, it does'nt work, as I said on my first post.