As I said above, I think its just a contrast between the accepted modern western incarnation of the Devil: i.e. the foreign Easterner Putin, and modern western incarnation of sainthood: the well known westerner (& multi millionaire) cuddly Attenborough, but as your following shrewd observation notes:
"...this particular image of the Attenborough vs Putin poster went viral on certain social media sites."
So... in effect here we have a demonisation of Putin in a way that is actually acceptable to a broader swathe of the Western public than the present form. All lefties and greenies can be induced into the anti Putin-Russia agenda if he can be painted as the arch enemy of the environment/planet rather than merely as at present: as an independent foreign competitor.
-It might augur a new development in anti-Russian propaganda.