When should I reference the name of the Library's photographer?
That depends on whether the photography was a basic reproduction or if there was some form of artistic creation involved on the part of the National Library's photographers.
In the Library's view, photographic imaging that involves no more than simple reproduction of an original does not qualify for copyright protection, meaning that there is no need to reference the name of the Library's photographer. For simple reproduction, it is sufficient to write “Reproduction: National Library of Sweden.”
On the other hand, if the graphic images did involve some form of artistic creative effort on the part of the photographer, for example in the form of lighting and image composition, they are considered to have copyright protection. To publish such an image, both the Library's and the photographer's name must be included with the image or in the table of images, as follows: “Reproduction: Name of photographer, National Library of Sweden.”