Ironworkers James Brady, left, and Billy Geoghan release the cables from a steel beam after connecting it on the 104th floor of One World Trade Center in New York on Aug. 2, 2012. The beam was signed by President Barack Obama with the words: "We remember," ''We rebuild" and "We come back stronger!" during a ceremony at the construction site June 14. Also adorned with the autographs of workers and police officers at the site, the beam will be sealed into the structure of the tower, which is scheduled for completion in 2014.

The name Antony is seen on a steel column on the 102nd floor of One World Trade Center on Jan. 15, 2013. Workers finishing New York's tallest building are leaving their personal marks on the concrete and steel in the form of graffiti.

A message left by Michael Chertoff, the former director of Homeland Security, on a steel column on the 104th floor of One World Trade Center, seen on Jan. 15, 2013.

Youths stand in front of a graffiti with Egypt's President Mohamed Mursi face on a playing card along Mohamed Mahmoud street near Tahrir Square in Cairo on Sept. 21. No sooner had Egyptian authorities painted over a wall of revolutionary graffiti near Tahrir Square this week than the street artists were back with spray cans and a new target President Mohamed Mursi.

Egyptian artists work on graffiti on a newly whitewashed wall in Tahrir Square, Cairo, Egypt, on Sept. 21.

A man draws graffiti along Mohamed Mahmoud street near Tahrir Square in Cairo on Sept. 21.

Egyptian artists work on graffiti on a newly whitewashed wall in Tahrir Square, Cairo, Egypt, on Sept. 21. Under cover of darkness, a few municipality workers quietly began to paint over an icon of Egypt's revolution: a giant, elaborate public mural on the street that saw some of the most violent clashes between protesters and police over the past two years. Artists have since worked to cover the whitewash with new art.

An Egyptian man waves the national flag next to a graffiti on a newly whitewashed wall in Tahrir Square, Cairo, Egypt, on Sept. 21.

Street artists work on their new pieces on a section of wall near Westbourne Park in north London on March 10. The group aimed to paint larger works that would look more attractive to residents than the "tags" and graffiti that were already on the walls.

Street artists begin creating new work on a section of wall near Westbourne Park in north London on March 10.

A street artist paints over old graffiti as his group begins work on new art on a section of wall near Westbourne Park in north London on March 10.

A graffiti piece by Shamsia Hassani and Qasem Foushanji on a wall in Kabul, March 5, 2012.

Two women walk by a mural depicting faces of Egyptians killed before and after the revolution, in Tahrir Square in Cairo on Dec. 20, 2011. The slogans read "No conciliation" and #NOSCAF, referring to the ruling Supreme Council of the Army Forces.

A girl, left, posts an art piece made by Sad Panda, unseen, on a wall as flower vendors prepare a bouquet outside their shop in Cairo on Jan. 19.

A man walks by a graffiti that reads "Pride and dignity, No SCAF," on a road that leads to Tahrir Square on Jan. 29.

Soldiers beat a protester wearing a niqab during clashes near Tahrir Square on Dec. 16, 2011. Graffiti in the background depicts members of the military ruling council and reads "Killer".

Graffiti written on the walls in Mohammed Mahmoud Street off Tahrir Square on Jan. 26.

In this photo taken Tuesday, May 10, street vendors wait for customers in front of a graffiti depicting Libyan dictator Moammar Gadhafi in Benghazi, Libya. After more than 40 years under Gadhafi, Libyans in the rebel stronghold of Benghazi have taken to mocking the Libyan dictator with colorful caricatures. Before residents in the rebel-held east ripped themselves free from Gadhafi's rule, drawing such satirical pictures of the leader in public was unthinkable, and the regime would have severely punished anyone caught doing so.

Artist Salhen Obaidi cleans his hands after painting a mural in downtown Benghazi, Libya, Sunday, May 15. Hundreds of new paintings and graffiti decorate the city of Benghazi since the rebels took control of the city.

In this Wednesday, May 11, picture a man walks next to a graffiti drawing depicting Libyan dictator Moammar Gadhafi in Benghazi, Libya. After more than 40 years under Gadhafi, Libyans in the rebel stronghold of Benghazi have taken to mocking the Libyan dictator with colorful caricatures. Before residents in the rebel-held east ripped themselves free from Gadhafi's rule, drawing such satirical pictures of the leader in public was unthinkable, and the regime would have severely punished anyone caught doing so.

A graffiti attributed to secretive British artist Banksy shows a dog urinating on a wall in Beverly Hills, California on February 17, 2011. Another graffiti was ripped down Wednesday in Hollywood, amid sightings of other pieces in a reported pre-Oscars publicity stunts. Banksy is nominated for best documentary for "Exit Through the Gift Shop" at the Oscars, due to be announced on Feb. 27 at the climax of Tinseltown's annual awards season.

An artwork painted on a billboard is seen in Los Angeles on Feb. 16, 2011. Street artist Banksy's first film "Exit Through the Gift Shop" is up for an Oscar -- and it seems the subversive Briton may be waging an unorthodox award campaign on the walls and billboards of Los Angeles. Several examples of graffiti bearing the hallmarks of Banksy's style and humor have turned up in areas of the city in recent days, including a Charlie Brown figure apparently bent on arson, and a cocktail-swigging Mickey Mouse.

A graffiti attributed to secretive British artist Banksy depicting Charlie Brown figure starting a fire on the side of a burned-out building on Sunset Boulevard in Los Angeles, California on Feb.17. Another graffiti was ripped down in Hollywood, amid sightings of other pieces in a reported pre-Oscars publicity stunt in LA. Banksy is nominated for best documentary for "Exit Through the Gift Shop" at the Oscars, due to be announced on Feb.27 at the climax of Tinseltown's annual awards season.

A graffiti attributed to secretive British artist Banksy depicting a child wielding a machine gun, in black and white surrounded by colored flowers, is spotted in Westwood, California on Feb. 17, 2011. Another graffiti was ripped down Wednesday, Feb.16th in Hollywood, amid sightings of other pieces in a reported pre-Oscars publicity stunt. Banksy is nominated for best documentary for "Exit Through the Gift Shop" at the Oscars, due to be announced on February 27th at the climax of Tinseltown's annual awards season.

A woman walks past graffiti on shopkeepers' shutters on February 8, 2011 in Madrid. The Spanish capital have invited graffiti artists to paint the steel shutters protecting their stores at night in the hope this will stop them from being defaced by crude scribblings made on the run. Over 130 graffiti artists from across Europe, including Bristol and Milan, decorated 140 shutters on February 6, 2011 in Madrid's central Malasana neighbourhood, which has long been the heart of the city's countercultural scene. Since painting over another graffiti artists' artwork, especially with lower quality work, is considered a serious insult by street artists, the shopkeepers believe they will no longer wake up to find their shutters covered with ugly, and sometimes obscene, drawings that can be expensive to remove.