She did not expect what followed: a Facebook phenomenon that gained half a million followers in a month and scathing, personal criticism by Iranian state television, accusing her of drug addiction, perversion and insanity.
Inspired by Alinejad's photo, taken in a public place with her hair showing without the Islamic veil that is obligatory in Iran, thousands of women inside Iran uploaded their own self-portraits to a page she hastily set up and called: "My Stealthy Freedom".
"To me, it was like a virtual demonstration on my Facebook page," the 37-year-old told Reuters in an interview, seeming genuinely astonished to find herself the figurehead of a campaign against Iran's restrictions on women's dress.
A political journalist who already had 200,000 Facebook followers before posting her selfie, she set up the separate "My Stealthy Freedom" to prevent her own page becoming swamped by women wanting to share their pictures.
"Look," she says, opening her laptop at a London cafe to show the most recent photo uploaded. "That was posted four minutes ago, and already has 439 likes and 11 shares."
Born two years before the Islamic Revolution that brought down the Western-backed Shah in 1979 and ushered in Iran's hybrid of democracy and religious rule, Alinejad is too young to remember her country before women were obliged to wear the veil.
As then, she says, many people underestimate the importance of the obligatory veil, saying there are far more pressing political issues. But she maintains that forcing a woman to cover her hair is the state's way of stamping its authority.
"When I was in Iran, my hair was like a hostage of the Iranian government."
"Our people's right"
Her Facebook followers agree. Many photographs show women standing in front of signs reminding women of their duty to respect the hijab, the Islamic dress code, holding their headscarves in their hands.
The photographs are unremarkable to a Western eye, but have outraged parts of the Iranian establishment which have hit back.
The state TV news channel IRINN, on a clip still available on YouTube, reported that Alinejad had been raped by three men, in front of her son, on the London Underground after she took her own clothes off while high on drugs.
The popularity of the page, and the vitriolic reaction, have made it the focus of one of the most prominent challenges to President Hassan Rouhani, a self-proclaimed moderate.
Like the arrest of six young people last month who posted a video of themselves - the women unveiled - singing along to the Pharrell Williams pop song "Happy", "My Stealthy Freedom" has shown the yearning of liberal-minded Iranians, many of whom voted for Rouhani, for greater personal freedoms.
"#Happiness is our people's right. We shouldn't be too hard on behaviours caused by joy," Rouhani wrote on Twitter after the "Happy" arrests.
He also appeared to agree that social rules - in a country where morality police patrol the streets to detain women they deem to be showing too much hair - should be eased, saying: "We can't take people to heaven by force and with a whip."
But reformist Iranians say those words have not been followed by policy changes. "All the nice words have expired," Alinejad said.
With Rouhani pushing for a nuclear deal with the West to lift crushing economic sanctions, and civil wars raging in Iran's regional allies Syria and Iraq, personal freedoms and women's rights are unlikely to be high on his agenda.
But Alinejad doubts Rouhani would ease the hijab rules even if he were able to in a system where the ultimate say lies with the supreme leader, Ayatollah Ali Khamenei.
And Alinejad does not trust Rouhani's invitation for all Iranian expats to return to Iran, where she fears she would be arrested due to her reporting on human rights in Iran that is carried by British and U.S.-funded broadcasters.
"I love my country and I never want to stay in England even for one single day if I am allowed to be safe in my country and cover the news that I cover from here," she says.
In response to the TV report of her drug taking and rape, Alinejad posted a video of herself, standing on the platform of the London Underground, singing a song in Persian about "my homeland", as people walk by, unfazed by her hair or her song.
"My real revenge was to use what the hardliners are most petrified of: singing a song without a veil, in London," she said. "Can you publish this video on Iranian TV? No. Would I be safe singing on the Tehran subway without a veil? No."
The state TV reported on the video - although it did not broadcast it - saying Alinejad had lost her mind due to the "rape".
© Thomson Reuters 2014