You get a photo of Nessie, the Loch Ness monster.
The actual Nessie photo is the THIRD photo you take at Loch Ness. On your first visit there, you get the camera and matches, and a rowboat ticket from the dart game at the Pub. You get a photo of humps in the water, and eventually take this to Mews to analyze. But the photo you get is of truck tires, and a shadowy Nessie.
On your second visit, you get a submarine ticket, but the photo you get is of a movie prop. After showing this at the Pub, you get the Pennywhistle that lets you call the real Nessie on your solo trip in the rowboat. This closeup photo goes back to Mews and confirms the existence of the Loch Ness monster.
After you have confirmed the other three cryptids (Nessie, Chupacabra, and the Jersey Devil), and brought back the inconclusive footprint from the Himalayas, Mews will send you in pursuit of Bigfoot, which is similar to the Yeti. Since you have him in a cage at the end (but let him out), he is definitely confirmed.
There are 3 you have to prove: Nessie (Loch Ness) Chupacabra (Puerto Rico) New Jersey Devil (New Jersey) You can't get what you need to prove the Yeti, so you are sent to find Bigfoot.
You still have to prove the chupacabra and the Jersey Devil. You can get a photo from the Himalayas, but it does not prove the Yeti. So when you have the other three proven, you will be sent to chase Bigfoot in the Northwest.
At Loch Ness, you are looking for the legendary monster, called Nessie. When you get tickets for the rowboat and the submarine, you can take photos that may prove its existence.
Prove 4 cryptids to qualify for the Mews million-dollar reward. You find a footprint that may be from the Yeti, but you can only prove Nessie, the Chupacabra, and the New Jersey Devil. Then you have to track Bigfoot and save him from Gretchen Grimlock. (see the related solution question)
You cannot prove the Yeti based on the evidence from the Himalayas. So you have to prove the other 3 (Nessie, Chupacabra, and New Jersey Devil) and when you do, Mews will send you to pursue Bigfoot in the Pacific Northwest.
Once you have gotten the lantern and the footprint photo from the Himalayas, you still cannot prove the Yeti. You can prove the other three cryptids (Nessie, the chupacabra, the Jersey Devil). Once you do, a fifth location will appear on the helicopter controls: you must go to the Northwest to chase Bigfoot.
You cannot prove the Yeti. But after you confirm the other three cryptids (Nessie, Chupacabra, and Jersey Devil), Mews will send you in pursuit of the similar creature, Bigfoot, in the Pacific Northwest. The location will be added to the GPS on the helicopter.
After you have the chupacabra tooth, you still need to prove that Nessie and the Jersey Devil exist. (The monks are stubborn about the Yeti and your snow footprint is unconvincing.) Get the photo of Nessie, and some egg shells from the Jersey Devil, and then you can pursue Bigfoot instead of the Yeti. (see related questions)
Run the DNA test on the eggshells to prove the existence of the Jersey Devil. You will also need to prove Nessie and the chupacabra, and bring back the photo of the footprint from the top of the mountain in the Himalayas. When all but the Yeti are proven (Nessie, chupacabra, and the Jersey Devil), Mews will send you after the Bigfoot in the Pacific Northwest.
You cannot prove the Yeti based on the evidence from the Himalayas. So you have to prove the other 3 (Nessie, Chupacabra, and New Jersey Devil) and when you do, Mews will send you to pursue Bigfoot in the Pacific Northwest.
You get the Island Medallion after you prove the existence of three of the four cryptids, and rescue Bigfoot from the evil Gretchen Grimlock. (see related question)