Mr Silver was a human man who was the manager at the Horizon Gallery.
Timeline 1
Sometime he hired Penelope to watch over the Paradox chamber. A short time later Mr Silver was attacked and killed by a creature.
When Penelope later realised that her older self was the creature and the paradox, she stoped herself from becoming that creature by sacrifying herself. (AUDIO: The Art of Death)
Timeline 2
Mr Silver hired Penelope to watch over an empty chamber. The Doctor suggested to Penelope to fill the chamber with her own paintings. (AUDIO: The Art of Death)