Why do the pictures of James & Lily NOT speak?

[dooddeb]

No: HPFGUIDX 121302


I was re-reading our Harry Potter books, (I share custody of them 
with my 12-year old), and realized the pictures in Dumbledore's 
office and Sirius' house not only move, but carry on conversations 
with the living.  Has it ever been explained why pictures of these 
dead people speak and the pictures Harry has of his parents do not?
Thanks!

-doobdeb