The Pokémon signs are special formations that must be found and photographed in order to get to Rainbow Cloud, where Mew can be found.
Kingler Rock
This sign can be found on the beach. At the beginning of the level, there will be several strange rocks on the left, but when viewed from the correct angle form a Kingler.
Pinsir Shadow
You can find this sign in the tunnel, about halfway through. There is a point in the tunnel where you will see a load of junk to the right. When the power plant is on, a silhouette of a Pinsir will appear behind the junk.
Koffing Smoke
You can find this sign near the beginning of the Volcano. When you begin, you will see some craters to the left. Throw a Pester Ball in one and you will see the smoke turn into a Koffing for a brief period of time.
Cubone Tree
You can find this sign near the River. At one bend in the river, you will see a cloud of Stun Spore created by sleeping Vileplume. The spore cloud is blocking the tree, so you must use the Pokéflute to wake the sleeping Vileplume in order to get a picture of it.
Mewtwo Constellation
You can find this sign near the Cave. In one area, if you look off in the distance to the left, in the same cavern as the bellsprout, you will see some purple, shining crystal-like objects. If you take a picture of this formation, you will see a strange portrait of Mewtwo when it develops.
Mt. Dugtrio
You can find this sign near the Valley. Just after ZERO-ONE stops near the river, look up to see a strange large mountain, that looks eerily like a Dugtrio.