There's been an old black-and-white cartoon of a tree swing that's been around for decades. According to one source, (presentation in pdf - see page 29) the original is pre-1970, and that's in accordance with my own recollection. According to another it is reputed to have been "First (?) seen in University of London Computer Centre Newsletter". There are many variants of the original.
But now someone's updated it in colour. Found while exploring Christoph Cemper's weblog.cemper.com.