In this heist, it's possible to steal the paintings without killing all the guards. But if one of them see any missing painting, will he trigger the alarm ?
While the guards will not be alerted if they don't see the painting where it belongs.
I highly recommend to be careful when moving the paintings because the cameras can be alerted to them.
Guards will be alerted if they see the painting on the floor.
Example: One time I threw a painting in the camera's range and it alerted everyone.
Mission Items in general are never detected to be missing when you take them otherwise day 3 for framing frames would require you to take out all the guards first.