The Conway's are appalled at the idea of Jamie and Paul having used their sofa as a platform to have sex on. So it would follow that if they were cleaning up the sofa, it would have been right after the party. And yet the painting is back in the frame. It's unlikely that the painting would even be found much less restored so quickly, Paul and Jamie live on the 11th floor. And since the painting was thrown out the window it would have been very hard to find.