The picture postcard was first introduced on 1 September 1894, but was not an immediate success. Early examples often showed seaside and city views rather than subjects or themes.
By 1902, however, things were moving; subject cards had been published featuring the Boer War and royal events, and in that year, the Post Office allowed both address and message to be written on one side of the card, freeing up the whole of the other for the picture. Britain thus became the first country to introduce the 'divided back' postcard format we are familiar with today. By this time, too, the size of cards had been largely standardised.
The Real Photograph Post Card (RPPC) is a cross-over point where the collection of old photographs and postcard collection meet. Essentially the card has a photograph on one side and a postcard back. Commercial photographers were quick on the uptake by offering their clients an option to have their portraits, wedding photos or other subjects printed onto photographic paper, the back of which was pre-printed as a postcard.

It would seem that the client (Victor Watson) permitted such use as a postcard. The card reverse shows that the photographic paper used is from a roll with pre-printed postcard back. As in this case (K Ltd) Kodak photographic paper manufacturer.