The artwork in question is on the left of the set if you are facing the TV, hanging above the bookshelf laden with books.
However, a lot of scenes on the long-running comedy are set in the same locations, including the living room set piece that Kaley Cuoco mentioned in the interview.
If you’ve ever paid attention to sets on The Big Bang Theory, you probably already realize there is a lot going on in the background, including various pieces of artwork, toys (action figures!) and books.