Hmm... perhaps I should edit that.

Ok... time to set limitations, just give me a moment.

EDIT: Here we go:
Artist cannot create beings that already exist in the world (ex: your characters, other people's npc's, etc.).
Artist cannot summon major gods/other similarly powerful beings for more than 1.5 seconds without falling unconscious (to the point where he's almost at a coma) for.... let's say 1 day.
Artist cannot create 2 of the same being at the same time, even if on different pictures.
If Artist returns anything to a picture, it cannot be summoned again without a cooldown time (let's say one hour).
Artist cannot summon things that violate the the basic laws of physics (ex. impossible objects).
If Artist himself is incapacitated (unconscious, etc) then all summoned things will be automatically returned to the picture they were summoned from.
All creations Artist summons will be subjected to the natural laws of the world.
Any creation too badly damaged/incapacitated will be automatically returned to the picture it was summoned from.

And this is already with his original limitations, like:
Artist cannot keep beings out in the world for long periods of time without tiring himself.
Artist can only keep a max of 7 objects/5 beings out at once (let's say the max amount of time is 1 hour for objects and half an hour for beings, and at the end of that, he'll be so exhausted that he needs several hours to recuperate).

Not quite done, but I'm running out of limitations as it is. If you guys would like to mention anything, then please do so, and I will consider it.