The Pokémon signs are very special, Professor Oak will give you an upgrade for The dash engine and whe you take all 6, you will unlock Rainbow Cloud where you can take a snap of Mew
Kingler Rock
This sign can be found on the beach, there will be lots of strange rocks but when viewed from the right angle form a Kingler
Pinsir Shadow
You can find this sign in the tunnel, there will be a point in the tunnel where there is loads of junk, but 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 Volcano, at the start, 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, the tree is behind this so use the Pokéflute to wake the sleeping Vileplume releasing the spores.
Mewtwo Constellation
You can find this sign near the Cave, in one area, you will see some purple crystal-like objects, if you take a picture here then 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, you will be able to see a strange large mountain, this is a Dugtrio with noses and mouthes