Did you just sent multiple images concatenated into the output stream, including file headers? And Mosaic would actually replace the first image with the second and so on? Was the CGI script referenced in an <img> tag?
What image format was this?
I think they were GIFs. From what I recall, I read the entire images as they were on disk and pushed them down the stream.
Someone mentioned https://en.wikipedia.org/wiki/MIME#Mixed-Replace and that is ringing a tiny bell.
Honestly, it was 23 years ago and I forgot the exact details. I still remember the meeting though. The feeling of dread, excitement, and ridiculousness of knocking out such a powerful machine (at the time) in order to server animated smiley faces.
Nowadays, I knock out powerful machines doing slightly more useful things.