From Bulbapedia, the community-driven Pokémon encyclopedia.
Pokémon signs (Japanese: ポケモンのしるし Pokémon symbol) are strange sightings found on Pokémon Island. Each bears an undeniable resemblance to a specific species of Pokémon. They are the only hints to the location of the rarest Pokémon on the island and one of the rarest in the world, Mew.
There are six signs in total, all of which are named by Professor Oak. In Pokémon Snap, taking a picture of one will result in Todd Snap being rewarded with the Dash Engine upgrade. Taking a picture of all six will result in Professor Oak realizing the connection between them: that each corresponds to a constellation in the sky, and therefore the rarest Pokémon must be found in the sky as well, on Rainbow Cloud.
Kingler Rock (Japanese: キングラーいわ Kingler Rock) is a sign found on the Beach.
It is an unusual rock formation which at first does not appear different from the other rocks in the course. However, from a certain angle, the pieces line up and it creates an image of a Kingler.
To view it from the right angle, the ZERO-ONE must turn around and watch the scenery behind it, waiting for the rocks to line up properly.
Pinsir Shadow (Japanese: カイロスかげえ Kailios Silhouette) is a sign found in the Tunnel.
Normally in the Tunnel, there is a point where pieces of junk and garbage are sticking up out of the ground in an unusual manner. While it looks suspicious, nothing can really be done with it unless the power plant has been revived. When this happens, a light turns on, shining through the garbage. The different shadows will combine to show the distinct silhouette of a Pinsir.
To turn on the power plant, Zapdos must be hatched from its Egg. This can be done with the help of Pikachu's Thunderbolt. Lure it close to the egg with Pokémon food, and then play the Poké Flute. The sparks will fly, the egg will hatch, the Zapdos will blast the power plant's generator with electricity, and everything in the Tunnel will come to life, including the Pinsir Shadow.
Koffing Smoke (Japanese: ドガースけむり Dogars Smoke) is a sign found at the Volcano.
Early in the course, craters can be seen off to the left of the path. Puffs of smoke rise from them. Sometimes the smoke will take on the expression and appearance of a Koffing. When this happens, it almost sounds as though a faint cry of a Koffing can be heard.
The smoke will change if a carefully aimed Pester Ball lands in the crater.
Cubone Tree (Japanese: カラカラのき Karakara Tree) is the sign found by the River.
At one bend in the River, set back from the bank a bit, there is a large tree which has grown into the form of a Cubone. However, it is usually hidden because a Vileplume often naps at its base, releasing a cloud of Stun Spore in its sleep.
The Vileplume will wake up to the sound of the Poké Flute. When it does, the Stun Spore will clear away and the tree can be seen.
The Mewtwo Constellation (Japanese: ミュウツーざ Mewtwo Constellation) is the sign found in the Cave.
In one large area of the Cave, there is a region off to the left where several purple crystalline objects can be seen. At first glance, their nature and purpose is not clear. However, when a picture is taken of this region, a ghostly portrait of Mewtwo develops.
Mt. Dugtrio (Japanese: ダグトリオやま Mt. Dugtrio) is the sign found near the Valley.
Shortly after the ZERO-ONE sets itself down on the river, a large mountain can be seen in the distance. However, it does not look much like any of the mountains around it. Instead, it looks like a giant Dugtrio, complete with giant eyes and noses.