It's JPEG, not PNG, but here you go, a demo by Kalman Reti (one of the Symbolics' developers):

https://www.youtube.com/watch?v=o4-YnLpLgtk

It shows a debug session where C code running on the lisp machine (VLM in this case, unknown variant of the VLM though) is debugged to fix JPEG decompression error.

Thanks, that's really intriguing. Ah, to think of all the things that could have been ..