Arguable — mass-mailing async updates (think: postcards) about your life has been ubiquitous for as long as people have been going off and doing interesting things. This evolved into mass-emails. I was BCC'ed a lot of "wedding photos.zip" and "in paris.jpg"s emails in the 1990s; much more often than such a thing was ever directed at me by a friend or relative, let alone directed at me synchronously in a messenger app.

Remember also: messengers back in the 90s and early 2000s didn't have a concept of "server-buffered message sends." For a message to transmit over AIM/ICQ/MSN/etc, both people had to be online at the same time. If you were travelling — and so potentially in a separate time zone — messengers really didn't work very well as a way to send large image files. Until very late in the lives of their protocols, most of them didn't even support sending files!