Does the library has a electronic subscription with National Geographic available for students?
We have a number of databases which provide "full text" National Geographic articles, but , as you have found, none of these provide the photographs, just the text of articles in HTML.
All publishers have the right to provide full text, or merely abstracts, in a format of their choice. A number do provide the articles as HTML, and this may limit their usefulness.
I am not aware of the business reasons which have motivated National Geographic to provide HTML- only articles, possibly the copyright of fhe photographs is owned by the freelance photographers who provide them. But you are correct - except for a few pictures visible on the National Geographic website with each article you do need to subscribe to see the full range of photography.