my wife, who knows everything, tells me that this is a sign used by fans of the film 'Free Willy' as a symbol of their ongoing commitment to animal welfare and environmental awareness.
What they don't realize is that defacing the currency in this way in this way is in fact a capital offence and as such is punishable by burning at the stake, beheading or garotting (you have a choice).
Of course, it might mean something completely different ~ my wife only thinks she knows everything.
It could for example be the secret call sign of a recently formed sect of animal fetishists ~ Frog Worshippers, perhaps.