I was not refering to any document, I was refering to my memory. As I mentioned, it never made it into an FTSC standard. Possible because there was no FTSC yet when it was used. It was used by Henk Wever's bbs/mailer Dutchie that was not used much outside The Netherlands. I am talking about the very early days of Fidonet. end 80ties, begin 90ties. Here is the picture that Henk used in his GIF kludge:
I merely meant to point out that having a kludge to facilitate displaying an image representing the author along with the message is not new. Indeed it differs from what you propose in that the GIF kludge directly gives the name of the file that contains the image.