Graffiti has been among the most powerful art forms and tools of Egypt's revolution and the turbulent months since, but it also has proven to be its most vulnerable and ephemeral.

For nearly two years, the slogans, portraits and artwork that went up on walls around the country depicting the goals, heroes and events of the uprising have been erased nearly as quickly.

So a group of artists, photographers and a publisher joined hands to preserve the images. "Wall Talk" — their newly released 680-page book — collects hundreds of photos of graffiti dating from the Jan. 25, 2011 eruption of the revolt against then-President Hosni Mubarak until today. The result is a street history that chronicles image by image the evolution of Egypt's upheaval, which has yet to settle.

In a sign of the continuing resonance of graffiti, the artists have recently turned to a new target: Newly elected Islamist President Mohammed Morsi.

Last month, a giant mural of revolution graffiti on a street off Tahrir Square, the focus of the revolution's protest demonstrations, was partially painted over, and within hours, artists refilled much of it with new images, some of them denouncing Morsi and his Muslim Brotherhood.

"You are a regime that is frightened by paint brushes and pens, you oppress and stamp on the oppressed. If you were doing the right thing, you would not be afraid of what's painted ... but you are a coward in your heart of hearts," reads a rhyming Arabic verse, addressing the president, one of the new images on the street, where some of the deadliest clashes between protesters and security forces took place last year.

But already the whitewashing has returned, erasing a portrait ridiculing Morsi, showing him with a smug smile and the inscription, "Happy now, Morsi?"

"Wall Talk" publisher Sherif Boraie says graffiti was the vehicle that delivered clear, strong and angry messages during the anti-Mubarak uprising and afterward. Now it reflects the depth of frustration over the perceived failure of the revolution to realize its main goals, he said. He sees the latest to go up as even angrier.

"We are in a difficult period, and the youth are very angry, while avenues for expression for them are limited," he said. "Will the anger continue to simmer indefinitely without boiling over? I don't think so."

The book includes a chronology of events of the past two years, but the images speak most strongly to the arc Egypt has taken. The pictures also show graffiti's increasing sophistication. Graffiti was almost never seen in Egypt during Mubarak's 29-year rule, where police kept a tight grip and where society generally frowns on street art of any kind. But it was the ideal medium for the leftist and progressive youth activists who led the protests against Mubarak.

"It's an important part of history," said one prominent artist whose graffiti appears in the book and who identifies himself only by his pen name "Ganzeer," Arabic for chain. "Many of the graffiti photographed and published in the book have been removed or painted over."

During the 18 days of massive protests against Mubarak's rule, much of the work was simply scrawled slogans, like the simple word "Erhal," the Arabic word for "Leave!" next to images of Mubarak.

After Mubarak stepped down on Feb. 11, 2011, the message and the images changed, with a colorful burst of optimism. There were the clinched fists symbolizing power and images suggesting unity and harmony between the nation's Muslim majority and Christian minority, with high expectations and a sense of confidence that if people power can bring down Mubarak's dictatorship, it can do anything.

"It's just the beginning," read the English words alongside drawings of women with long hair in the black, red and white colors of Egypt's flag on one Cairo wall, shown in the book. The exuberant slogan "Hold your head high, you are Egyptian" was found on walls around the capital. One graffito was written on the pavement, reading: "Don't look down, freedom is ahead of you."

But very quickly the word on the walls turned to revolt once more — this time against the military council of generals that took power after Mubarak's ouster and ruled until Morsi was elected this year and was inaugurated in late June. Protests repeatedly broke out against their rule and were met by bloody crackdowns.

Graffiti portraits mocked the council's head, Field Marshal Hussein Tantawi — who was later removed by Morsi — and other generals, unprecedented in a nation where the military has held immense powers since a 1952 coup and was considered above questioning.

Throughout the past 19 months, graffiti has also sought to keep alive the spirit of the young activists' vision of the revolution despite continual setbacks — including by enshrining their heroes.

Slogans often went beyond current events to encourage change in a society accustomed to dictatorship. "The truth is not cruel, but freeing oneself from ignorance is as painful as going into labor," one cries out. "Chase after the truth until you are breathless. Endure pain so you can be born again."

A common theme was elaborate portraits of the "martyrs," the hundreds of protesters killed in multiple crackdowns. Some shown in the book are idealized, even giving them an angel's wings. Others depict them casual and smiling, as if they are standing right next to you.

A frequently depicted heroine is Samira Ibrahim, a young woman who went public with accusations that soldiers conducted humiliating "virginity tests" on her and other detained female protesters. One image shows her face, in her conservative headscarf, over rows of soldiers, proclaiming, "Above the military."

"Graffiti has won us freedoms we had never dreamed of before," said Mohammed Hashem, a prominent publisher whose office in downtown Cairo has been among the most favorite meeting places for leftist revolutionaries. "It has been the strongest voice of the revolution."

Ironically, graffiti has also broken into an Egyptian art world long dominated by elites who tended more to traditional landscapes or abstract art.

In a first, Ganzeer adapted his graffiti work to traditional tools — oil on canvas, wood or water colors — and took it to a gallery in Cairo's upscale Zamalek district this week. He called the show "The Virus Is Spreading," a name he said he chose to suggest the spread of graffiti from being a street art to a genre that could win acceptance and respect.

"It is an art that is totally different from the art sanctioned by the Mubarak regime," said Ganzeer, whose work is on offer for anywhere between $400 and $5,000.

Another prominent graffiti artist, known by his signature of Sad Panda, exhibited alongside Ganzeer. But he sees attempts to preserve graffiti as, at least in some ways, conflicting with the genre's very nature. Like Ganzeer, he does not give his real name to the public for fear of retribution.

"Every art form has its rules. When I paint on wall, I commit my art to the street. The street owns it. The street and whoever in it can do what they want with it," he said. He shot to prominence with his images that always included a big-bellied panda with a sad but pensive face. In reality, he recounts, he was called panda by school friends because he was overweight, and he added the "sad" because it reflects what he describes as his "black disposition."

"To me, politics is absurd, stupid and sad. It is all about winning power," Sad Panda said. "But I did take part in the revolution. I cannot be living in a nation that has a revolution and not participate."