I'm always skeptical when there is assertion that a type of story that has been around for millennia "became popular" in a specific time. There is no question that they "were" popular in the nineteenth century, but it is equally clear that they were ubiquitous in the written record throughout history.

Is there an assertion that photography contributed to this? I'm not sure, so let me know if there is such an assertion and what its context is. There were spirit photographs that became a popular device in the spiritualist movement of the second half of the century; these were generally understood to be created by the photographer and were kept as sentimental keepsakes that make it seem that the departed was hovering above, looking out for the living.

Despite the use of photograph in this context, the Romantic movement included a fixation with the dead that well predates photography. Gottfried August Bürger (1747–1794) wrote his famous poem, "Lenore," in 1773, dealing with the return of a dead lover who takes his would-be bride to his grave. This poem is regarded as a benchmark for the Romantic movement. It encapsulates the movement's fixation with death and mortality, themes that tugged at heartstrings in "romantic" ways.

Driving all of this even further into the past, it is important to point out that the story that inspired Bürger was a very popular legend, classified as ATU 365 and named "The Lenore Legend" in honor of Bürger's masterpiece. But the point here is that the legend is very old and had been popular for centuries before the poet used it for his inspiration. There is even a pre-conversion version of it that appears in the Elder Edda collection of Scandinavian poetry. I deal with all of this in my article, "'The Spectral Bridegroom': A Study in Cornish Folklore", which I later adapted to serve as a chapter in my book, The Folklore of Cornwall: The Oral Tradition of a Celtic Nation (2018). The point here is that the popularity of this legend was considerable for many centuries; it then inspired an eighteenth-century poet who helped define a movement that included a fascination with death as one of its cornerstones. The fluidity of the history behind this is not to be underestimated, and in all of this, ghosts play a big, consistent part.

I have heard it asserted that the considerable death toll of the American Civil War (1861-1865) created as widespread American yearning for contact with the dead, giving some energy to the Spiritualist movement. Again, making this connections is always problematic since we take two events - the War and the movement - and we then create a casual relationship between the two. Hard to say, but it is possible.

For whatever reason, the Spiritualist Movement was popular on both sides of the Atlantic and it included a great deal of interest in the afterlife and in proving that survival of death occurred. There was photographic evidence that was occasionally used - photographs that depicted hazy forms where no one had seen one. These glitches in the photographic process were interpreted by believers to be evidence of ghosts, but the photographs did not create the fixation; instead, they were used to provide evidence. In the same way, Spiritualist Movement advocates used the Cottingley fairy photographs as evidence of another type of spirits in our midst - in this case fairies. We would not, however, say that the photographs created the belief or the enthusiasm since belief in fairies had been popular and widespread for centuries.

A bit rambling - sorry; but these observations may give you a place to hang your hat.