It’s a heroic image: an indomitable woman taking to the city streets, offering herself as a symbol of courage, thrusting her right arm upwards into the air to lead the charge against the encroachment of ultraconservatism. Anyone not logged into social media this week could be forgiven for thinking I’m describing one of most inspiring political paintings in the history of art: Liberty Leading the People (1830) by the French Romantic artist Eugène Delacroix. In fact, the iconic gesture appeared in real life last Sunday in a central Swedish city, and was captured in a photo that has gone viral the world over.  When Tess Asplund, a 42-year-old Afro-Swedish social activist, came face-to-face with a May Day march of 300 uniformed nationalists in Borlänge, she felt compelled to register her disgust with the group’s anti-immigration attitudes. Unaware her actions were locked in the lens of a spectator’s camera, Asplund strode into the street and placed herself, like a human speed bump, in the parade’s path. With fist clenched and arm raised at an angle that recalls the Black Power salute, Asplund unwittingly found herself posing for a photo that would spread like wildfire on Facebook and Twitter in the ensuing days.  Though entirely unstaged, the viral image has all the aesthetic power of Delacroix’s epoch-defining painting. That shows the figure of Liberty jabbing the tricolor flag of the French Revolution into the gunpowder air and allegorically leading an impromptu army of civilians in protest against the authoritarian policies of King Charles X. Though the Bourbon monarch may not have been preoccupied with curbing the influx of immigrants, the July Revolution of 1830 that resulted in Charles X’s toppling was nevertheless ignited by grave concerns about the King’s sudden lurch towards ultraconservative principles, including his ambition to clamp down on the free speech of journalists.  Both images – the photo of Asplund and the 19th-Century painting of Liberty that it echoes – rely for their visual intensity on the pyramidal composition of their figures. In Delacroix’s canvas, the raised arm of Liberty and the flagpole she grips establish the two sides of a triangle that our eyes must constantly climb and descend as they rehearse the ups and downs of the noble struggle.  In photographer David Lagerlöf’s picture of Asplund, the recast heroine is captured in profile: yet the angle of her outstretched arm and the line created from her fist downwards diagonally through the heads of the marching nationalists reasserts the structure we remember from Delacroix’s painting. The suggestion of a triangle – the strongest shape in geometry – is part of the photo’s archetypal power.  100 Works of Art That Will Define Our Age by Kelly Grovier is published by Thames & Hudson.  If you would like to comment on this story or anything else you have seen on BBC Culture, head over to our Facebook page or message us on Twitter.  And if you liked this story, sign up for the weekly bbc.com features newsletter, called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.
A photo that has gone viral this week shows a single figure standing up against nationalists at a rally in Sweden. Kelly Grovier looks at how the image sums up resistance.