What is the painting of in Alfred Hitchcock's PSYCHO that covers the hole in the wall?

"The painting that Norman removes from the wall to spy on Marion undressing is a replica depicting the biblical story of Susanna and the Elders; it is about a virtuous young woman who is spied on by two old men while she bathes. The elders then try to blackmail her into having sex with them."

It's a version of the biblical story of Susanna and the Elders.
This is a painting based on an original ivory carving by Francis von Bossuit, 1690
https://commons.wikimedia.org/wiki/File:Susanna_and_the_Elders,_ivory_sculpture_by_Francis_van_Bossuit,_Getty_Center.JPG
The artist who made an engraving based on the sculpture is identified by the RMN as Frans van Mieris the Elder (1635-1681), and the work (1731) is said to be at the Hyacinthe Rigaud museum in Perpignan. However, another French government site attributes the painting to Willem van Mieris (1662-1747), Frans's youngest son:
https://www.pinterest.com/pin/230035493439392409/

Read more here:
http://www.thecinetourist.net/a-picture-of-great-significance.html