This was not a genuine photograph of a mermaid, but a digitally altered image of a beached whale.
It was created by Eduardo Valdés-Hevia, a digital artist whose Twitter bio states "my pictures are slightly edited."
Here's a comparison of the doctored mermaid image (left) and the genuine photograph (right):
The genuine photograph on the right comes from the "Picturing Canada" collection, a series of photographs taken between 1895 and 1923 that were digitized by the British Library in 2012 and made available via Wikipedia. Details for this specific image are scarce. The only information available comes from the title: "At River John, Nova Scotia, August 1918. [Beached whales.]."
The digital artist who created this mermaid image has made other similar artworks. The following image, for example, supposedly shows a group of people wrangling some sort of giant bug on a beach:
Again, this is a digital artwork and not a genuine photograph. This "bug" image was, again, created by altering a genuine photograph of a whale. Here's a comparison of the digital artwork (left) and the genuine photograph (right), which shows a group of men peeling blubber from a whale carcass in 1907, according to the Library of Congress: