That painting was by Francis Bacon and titled Figure with Meat, which is owned by the Art Institute of Chicago. On this work, the website of the Institute notes:
Permeated by tormented visions of humanity, Francis Bacon’s paintings embody the ethos of the postwar era. Beginning in the late 1940s, Bacon created a series of works modeled on Diego Velázquez’s Portrait of Pope Innocent X (1649/50), in which he transformed the celebrated masterpiece into grotesque, almost nightmarish compositions. In this version, he replaced the noble drapery framing the central figure with two sides of beef, directly quoting Rembrandt van Rijn and Chaim Soutine’s haunting images of raw meat. By linking the pope with these carcasses, Bacon allowed the viewer to interpret the pope alternately as a depraved butcher, or as a victim like the slaughtered animal hanging behind him.(Internal links added). No wonder the Joker saved the painting from destruction.
Inspired to paint by Picasso, Bacon was a painter of grisly images. How to describe his ouevre? Writes Steven Litt of the Cleveland Plain Dealer: "His paintings of screaming popes and caged businessmen - icons of the modern age - seem to flay the skin off their subjects and mold flesh like raw clay. His portraits lacerate foreheads, crush cheekbones, warp eyes and lips out of alignment and transform torsos and limbs into pretzels of twisted meat."
Figure with Meat, painted in 1954, has been called a "notorious riff" on Velazquez's Portrait of Pope Innocent X.1 Mary Louise Schumacher of the Milwaukee Journal Sentinel offers a bit more detailed analysis of the work, its homage to Velasquez, and its depiction of the pope:
Bacon appropriated the famous portrait, with its subject, enthroned and draped in satins and lace, his stare stern and full of authority.In sum, the painting "exhibit[s] Bacon's fascination with the screaming mouth set against a gray face that seems to be in the later stages of necrosis, with wrinkly eyes peering out helplessly from behind a mask of decaying, translucent flesh."3 Litt, in the piece linked above, observes that Figure with Meat is "less an assault on Catholicism per se than an image of an authority figure reduced to utter anguish and helplessness."4
In Bacon's version, animal carcasses hang at the pope's back, creating a raw and disturbing Crucifixion-like composition.
The pope's hands, elegant and poised in Velázquez's version, are rough hewn and gripping the church's seat of authority in apparent terror.
His mouth is held in a scream and black striations drip down from the pope's nose to his neck. It's as if Bacon picked up a wide house painting brush and brutishly dragged it over the face.
The fresh meat recalls the lavish arrangements of fruits, meats and confections in 17th-century vanitas paintings, which usually carried subtle moralizing messages about the impermanence of life and the spiritual dangers of sensual pleasures. Sometimes, the food itself showed signs of being overripe or spoiled, to make the point.
Bacon weds the imagery of salvation, worldly decadence, power and carnal sensuality, and he contrasts those things with his own far more palpable and existential view of damnation.2
See for yourself (and click upon the image to enlarge it):
The painting with which Bacon became so enthralled, Velasquez's Portrait of Pope Innocent X, is below (and can be clicked upon for a larger image):
Figure with Meat recently toured a number of museums as a part of the "Francis Bacon: Paintings from the 1950s" exhibit (more about which can be found here and here). In fact, a television advertisement for the exhibit survives on the YouTubes and can be seen here:
The Joker defaced a number of paintings in that sequence (though I've been unable to locate an online listing thereof), but he did append his signature to one work for good measure:
Figure with Meat has drawn a number of blog and Usenet comments over the years (mostly prompted by its appearance in Batman, including some such posts here, here, here, here, and here. It seems that Tim Burton can be thanked for generating some level of interest in Bacon among those previously unaware of his work or existence. So, last but not least, behold the sequence from Batman featuring Nicholson, Kim Basinger, and various members of the supporting cast that inspired all of those inquiries about the Bacon painting:
1. Dorothy Shinn, "85 Drawings at Oberlin Mark Ohio Son's Return; Jim Dine's Portraits, Figure Studies and Pastels in 40 year span on Display," Akron Beacon Journal, June 26, 2005.
2. Mary Louise Schumacher, "Screaming in Paint," Milwaukee Journal Sentinel, January 27, 2007.
3. Colin Dabkowski, "The dark side: An Albright-Knox exhibit of Francis Bacon's paintings provide a chilling study of the violence that seemed unending in the 20th century," Buffalo News, May 8, 2007.