What do you do after you get the picture of the lochness monster?

Take it back to Mews, who dismisses it as a snowshoe print. Once you have the lantern and the footprint photo, there is nothing else to do about the Yeti.

However, once you have proved the other three cryptids (Nessie, chupacabra and the Jersey Devil), Mews will send you in pursuit of Bigfoot instead.