I wonder if Ariana's portrait's inability to speak was related to her mental problems. I don't remember if it was fully explicated, but I could see the actual Ariana not speaking very much or at all - particularly if there were strangers around. It's also definitely possible that whatever spell they did to make her portrait, didn't allow for speech.

Which makes me wonder if Snape's portrait could talk. Oh man, a silent Snape portrait, that would be terrible. He just glares at Everyone, Constantly.

I can imagine the Ministry not accepting Umbridge's portrait, even if it was donated to them upon her death. She doesn't seem like the type to have family... I wonder if it's possible to destroy a portrait...