Photographs of the endangered Magnolia dealbata, held by Bristol-based Arkive, were seen by a botanist at a Mexican university more than 5,500 miles away.
The plants were tracked to Mexico's Serra Gorda Biosphere Reserve and identified.
Lucie Muir, chief executive of Arkive, said they were "thrilled" by the discovery.
After seeing pictures of the flowers, botanist Dr Jose Antonio Vazquez asked the original photographer for more images of the plant.
Photographer Roberto Pedraza Ruiz said he then made several more trips to the forest "documenting the flowers and fruits of the trees until finally receiving confirmation that I had photographed not only one but two completely new species of magnolias".
The first specimen was named Magnolia rzedowskiana, after emininet Mexican botanist, Dr Jerzy Rzedowski, while the second is to be named Magnolia pedrazae, in honour of its photographer.
Mr Ruiz said: "This is without doubt the highest honour that a conservationist and nature photographer can receive."
The Arkive website, run by the charity Wildscreen, hosts 16,000 images of flora and fauna.
It claims to be the world's largest online encyclopaedia of life, receiving donations of images from wildlife film-makers, photographers, conservationists and scientists.

Laura Turner sent a series of lewd messages to one of her co-workers while on duty, the Health and Care Professionals Council tribunal heard.
Staff at the University Hospital of North Durham said her comments had a "devastating" effect on the department.
Ms Turner said she was "disgusted" with her actions.
During the hearing in Kennington, South London, one of the victims said in a statement she was angry and in disbelief and "went to have a little cry" after the incident.
Ms Turner's line manager Judith Allen described the messages as humiliating and degrading and said the department was shocked at the content and her "unprofessional behaviour".
Ms Turner, who did not attend the hearing, was dismissed after the Facebook comments were shared in March 2014.
Panel chairman Philip Geering said: "The panel is of the view that the registrant's conduct fell well short of what would be proper in the circumstances and breached the standards of the HCPC."
He said sending messages that were sexually explicit and degrading to her colleagues while on duty in a clinical setting made her actions "even more unacceptable".
During the tribunal, Ms Turner was described as having a "cavalier" attitude to patient confidentiality, after it was revealed she shared photographs with X-rays and patient notes visible in the background.
Mr Geering said her "sufficiently serious" actions posed a "significant risk" in undermining public confidence in the trust and in the health care profession.
Ms Turner accepted writing and sending the comments during an interview as part of an internal investigation, the hearing was told, but claimed she was "easily led" and said her colleague "told me what to write".
Mr Geering said she had failed to show "meaningful remorse" and was "focused on the impact on her personally and the risk of losing her job".

Munashe Charles Kutyauripo, 16, known as Charlie, was attacked at Ashton Playing Fields following his friend's party at Woodford social club.
Police said the boys were arrested at separate addresses and were in custody.
Detectives are continuing to search for 16-year-old Aaron Alexander Tavares-Gaiete.
Detective Chief Inspector Mark Lawson said Charlie's family had been left "devastated" by his death, and "wanted and deserved answers" about what happened.
His family said in a statement: "We hope that if anybody knows anything, they should come forward and help find whoever did this because to the world he was just a 16-year-old boy, but to us, that was our son taken."
A post-mortem examination found he died from a stab wound to the chest.
Two new species of magnolia flower have been identified after being spotted on "Noah's Ark" online archive.
[NEXT_CONCEPT]
A radiographer who "humiliated" her colleagues by posting sexually explicit comments on Facebook has been found guilty of professional misconduct.
[NEXT_CONCEPT]
Two 17-year-olds have been arrested on suspicion of murdering a teenage boy who was stabbed after a birthday party in east London.