Night Sky #2. 1991, oil on canvas. By Vija Celmins.
Quote: With jewellike imagery and a suggestion of vastness, Night Sky #2 is at once romantic and unsettling. Without the anchor of a horizon or a humanly scaled reference point, viewers may find it difficult to determine their relationship to the image. As a result, they may feel simultaneously engaged and distanced in the presence of this mesmerizing view.
Or, as a result, they may snap and slash it to bits. Via IO9.

I think the guard was expressing a preference for the works of Lucio Fontana.