[Alexinator]

Thanks 

[Randomizer]

Well, in order to unlock every pictures in the album, you must collect every single dreamstone in that particular level. Also, defeat every giant moo you see and they will drop some dreamstones since they are counted too. Miss one and you'll have to do the level all over again.

EVERY. SINGLE. ONE.

[VikVanHazel]

Actually, you need to collect at least 150 dream stones in each stage. It is actually possible to get more than 150 dream stones in a small number of stages! But for the most part, you need to collect every dream stone you can find!