MtG - when i have Cold Storage in play, can I put creatures in my graveyard on it?

No, because there are no 'Creatures' in the graveyard. 'Creatures' are things that only exist on the Battlefield. When they are in the graveyard, or other zones, they are 'Creature Cards'. That is a subtle but major distinction, and it's the reason why a lot of cards don't specify the zone they affect - they don't need to.

For example Unsummon doesn't say it can't be used on something that's being cast, and on the stack, but there they are 'Creature Spells' and not 'Creatures' so are not valid targets. Likewise it doesn't say it can't be used on something in the graveyard, but as said already, these are not 'Creatures', they are 'Creature Cards'.

Therefore since Cold Storage only targets Creatures, the only valid targets will be on the Battlefield, you can't target anything in other zones.

Cold Storage specifically says "Creature you control".

Creatures in your Library, Hand and Graveyard are not "under your control". That applies only to Creatures which are on the Battlefield.