In his death portrait, the young rebel’s bearded face is fixed with a broad, unearthly grin. The Saudi man had been killed in fighting, and his corpse, with its beatific smile, was photographed and displayed in a Twitter posting inviting others to celebrate his martyrdom.

“He always used to say: ‘Those martyrs smile. What is it they see?’ ” a former comrade wrote in a tribute to the fighter, identified as Abu Hamad al-Saya’ri. In another post, an admirer mused about the good fortune of the fallen and speculated on what the dead must be saying: “Congrats to me, congrats to me, I became a martyr.”

The Saudi fighter is one of hundreds of Islamist veterans of the Syrian conflict whose deaths are heralded in Web postings, many of which feature bloody — and occasionally smiling — portraits of the newly deceased. Although the images may strike many Westerners as macabre, they have become one of the rites of service among Syrian jihadists, as well as a popular recruiting tool.

“These guys are celebrated, and to young people back in the neighborhoods, they are heroes,” said Steven Stalinsky, executive director of the Middle East Media Research Institute, a Washington nonprofit group that monitors websites and news media in the region. “They look at the photos and they say, ‘I can be this guy.’ “

The memorials for dead fighters are but one manifestation of an explosion in the use of social media by Islamists since the start of the Syrian conflict in 2011. Although jihadists have long used the Internet to communicate messages from leaders or spread images of battle, Syrian rebel groups are flocking to websites such as Twitter, Instagram and Flickr to create new ways to recruit, train, raise funds, debate theology and coordinate strategy, researchers say.

To some Syrian rebels and their supporters, Twitter is not just a communications tool but also an online cash machine, useful for soliciting donations or even running auctions for donated cars and jewelry. Others use Skype accounts to conduct interviews with potential recruits or to share advice on military tactics. Still others employ YouTube or Facebook to trumpet their battlefield successes or to document alleged atrocities by their opponents.

The sites’ growing popularity among the more extremist Syrian groups — including some with ties to al-Qaida — has prompted calls for more stringent policing by the U.S. companies that own them. A handful of accounts have been blocked, but the vast majority continue to operate, providing a rare window into the thinking, planning and tactics of jihadists groups in Syria and beyond, analysts say.

“What’s interesting in the age of Twitter is that the normal jihadi cultural material — previously confined to private discussion forums — is being broadcast more widely where everyone can see it,” said William McCants, a Brookings Institution scholar who studies the Islamists’ use of the Internet. “The closed jihadi community is coming out into the light of day and inviting others to join.”

Among the most striking features of the Syrian conflict is the profusion of social-media postings devoted to individual fighters, both living and dead.

Since the arrival of the first foreign jihadists in Syria more than two years ago, rebel volunteers have used Facebook and Twitter accounts to keep their friends and relatives updated about their experiences, just as U.S. troops did in Iraq and Afghanistan. When fighters are killed, the same websites offer a way to spread the news to family and friends and pay tribute to the fallen, researchers say. With flowery language and photographs that sometimes border on ghoulish, the online tributes seek to reinforce the Islamist belief in martyrdom as a high calling — a noble death that guarantees a place in paradise.

Last year, Stalinsky’s group began to catalogue the “martyrdom” tributes showing up on Facebook and Twitter. But a year later, Stalinsky said, there were so many postings that the organization could no longer keep up.

Many of the postings include images and allusions intended to resonate with the Muslim faithful. In some photos, bodies with grievous wounds are posed so that they appear to be smiling or, in some cases, pointing to heaven. Dead rebels with wide, toothy grins elicit admiring comments, as was the case when the image of a lightly bearded youth was displayed on a Twitter account in April.

“We have seen many martyrs smiling when they meet with their God, but we haven’t seen a smile this wide,” responded a man writing under the Twitter name FahadJabbar1. “What did he see to make this beautiful smile? Oh Allah, grant us martyrdom.”

A common belief among jihadists is that martyrdom brings special rewards in paradise, including the affections of the “houris”_ the 72 black-eyed virgins promised to men in the afterlife — as well as an ability to win entry to heaven for the martyr’s relatives.

Jihadist teaching also holds that the martyr’s corpse resists decay and exudes a perfume-like odor. Thus, many of the postmortem photos are accompanied by testimonials claiming that the bodies possessed an otherworldly aura.

“The scent of musk emanated from his pure body, and it was smelled by the mujahideen,” Twitter member Albaraibnmalik1 said of a dead fighter whose smiling, bearded face was posted in March. “Women trilled loudly after they smelled the musk emanating from his body.”

Until recently, when a terrorist leader such as Osama bin laden sought to convey a message, he would send videotapes via couriers to trusted assistants who would post them on Islamic websites after delays of several days or even weeks. By contrast, rebel commanders in Syria have instant access to thousands of followers.

Groups in Kuwait that support the Syrian rebels have used Twitter to run increasingly elaborate fundraising drives, including online auctions that accept cash bids for donated luxury goods, including Land Rover SUVs, diamond necklaces and resort properties.

Others use social networking sites to distribute not only resources, but also ideas. Some of Syria’s larger rebel groups such as Jabhat al-Nusra, an Islamist faction linked to al-Qaida, use Twitter and Facebook in much the same way that corporations and government agencies do in the United States. The groups run sophisticated media outreach programs that allow them to better manage their public image, said Rita Katz, executive director of SITE Intelligence Group, a private firm that monitors jihadists’ Web postings.

Jabhat al-Nusra, which has been labeled a terrorist organization by the Obama administration, has sought to counter that image with an aggressive social-media campaign that seeks to cast the group in a softer light, Katz said. Some recent postings on YouTube showed Jabhat al-Nusra fighters handing out food and blankets or clearing trash from rubble-strewn Syrian neighborhoods.

“Clearly this new style of media propaganda is very different from the old fighting images affiliated with the jihadist outlets,” Katz said. Such efforts reflect an ambition to “win hearts and minds of the local population as well as the international community,” she said.

Such messaging depends on services and networks run by U.S. firms. Twitter, the San Francisco-based firm, is the preferred platform for many of the groups in Syria. It is also regularly used by top al-Qaida terrorists, including Ayman al-Zawahiri, the group’s leader since the death of bin Laden. Twitter in recent months has taken steps to shut down a number of accounts linked to known terrorists, but company officials have said that they lack the resources to monitor the estimated 500 million registered users and 340 million messages posted on a typical day.

Although attempts to police the sites are laudable, Katz said, “in most cases, they fail to make a significant dent.”