OOTP question: How do the paintings work?

[xpectopatronum]

No: HPFGUIDX 62460

Hello,
I have another question about ootp. 
How do the paintings work? All the people in the portraits are dead,
that's obvious, so how do their portraits 'stay alive'? Perhaps there
is a portrait of Sirius somewhere, and Harry will be able to talk to
his 'portriat', as if it were him? But I'd still like to knowthe
theory behind it all.
Do only certain people get their portraits painted,and if so, who?
People with money, and/or people in a position of power it would seem.
But then, what about the Fat Lady?
And what do you need to do to a painting to make it come to life?

-Cindy