At the end of each course, you have to pick one picture of each Pokémon that you will show to Oak. He judges them by the following five categories and gives you the total score.
Special Pictures: Some pictures are special pictures that Oak gives a name to. Examples: "Surfing Pikachu", "Pikachu on a Stump", "Graveler's Group Dance", etc. Unlocking these special photos usually requires precise timing and the use of unlockable items. To find how to snap these, review each course in the walkthrough.
Size: The bigger the Pokémon is in the photo, the higher your score will be. However, be careful not to be too close to the Pokémon, because if part of the Pokémon sticks out the edge of the photo then the score will be lower. 1000 is the best score you can get for size.
Action: Idle Pokémon are worth little. Try to influence its behavior by using one or more of the unlockable items to get a reaction.
Position: the score is doubled if the Pokémon is in the center of the frame.
Same Pokémon: If you get more than one of the same Pokémon in the picture, Oak gives you a small bonus depending on how big or how many extra Pokémon are present.
Extra rules: Never take a picture of a Pokémon from behind as it will either not be identified at all, or score very little.