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.
Two new species of magnolia flower have been identified after being spotted on "Noah's Ark" online archive.