Yes Ryan, sorry... my bad. Direct links work just fine.
To make it work on php4 (no caching) I had to give up on all file() or file_exists() or is_file() conditions to check the remote files existence. Actually, that was the reason I got scared in the previous post.
When caching is disabled, I wanted to prevent the original gravatar link being dead (like when Gravatar - Globally Recognized Avatars server is down). It works fine on php5 but not on php4, so I guess I'll take that risk of having broken gravatars from time to time (on php4 only).

I'll make a few more refinements next days, then I'll post you a link to a "gravatarized" phpMyChat-Plus version ;)

Thank you Wildhoney and Ryan for your very helpful work.

PS: I'm so excited now that things started working as expected!