Painting over cracks in ceiling
I encountered this in my first property, which due to the age of the building would have cracks in many walls and ceilings.  

In my experience paint does not do the job.  Perhaps if you slapped a lot of thick, gloss on top of them you could "fill" them, but I don't believe standard interior matte paint will reduce visibility of the crack.

In an ideal world, you would tape over them and use some sort of jointing/finishing compound to cover the area approx 5cm either side of the crack along it's length.  A bit of sanding and paint and you're sorted.

However, with your ceiling this is either going to be impossible, or look very obvious after the fact.  

Advice you don't want, but will definitely fix it: re-plaster the whole ceiling

Cheaper, but might not work: run some caulk (with the smallest nozzle possible) down the crack and smooth in with your finger.  Wipe away the excess and let it dry.  Paint over.  It may not be perfect, but it has the potential to work in the long-term as the caulking is more flexible and will allow for a little movement.