He might have run that program himself if it was a fairly large sheet or two of fanfold paper.
Way back when I learned programming in the days of keypunch cards we did stuff like that on IBM mainframes. It took a couple of hundred cards to make such an image if it's what I think you are talking about. This was way before the internet so you couldn't just trade your programs unless you shipped off a bunch of cards to the person looking for the program.
It's possible that some of this was in some magazine but all the one's I remember doing were all coded by me or friends.
We coded the cards to print a character at a specified position on the page and once all laid out you had art!
This link might shed some light on what I'm thinking of.
http://www.geocities.com/joan_stark/textasciihistory.txt
Kevin