The concept of photomicrograph it can be used in different ways. The first meaning mentioned by the Royal Spanish Academy (RAE) in your dictionary refers to the technique applied to reduce the image of an object.
The idea of photomicrography can also refer to the Photography which, appealing to a camera with special characteristics, is taken from a microscopic object (that is, very small). When the photomicrograph is made of something that is observed through an electron microscope, we speak of electron photomicrograph.
French inventor and photographer René Dagron (1819–1900) was the one who patented for the first time a process that allows obtaining microphotographs in the first of the aforementioned senses. With his technique small images of much larger items could be achieved.
Regarding the possibility of portraying microscopic objects, one can venture into photomicrography with a digital camera, a microscope and an adapter. In this way, images of tiny pieces are recorded.
Photomicrography, in this framework, can be associated with micrography: that’s the name of the description that is made of what is observed through a microscope. When what is observed is captured in an image, it is a photomicrograph.
It is important to mention that there are those who call this process photomicrography or microscopic photography. According to this position, the photomicrograph would be exactly the opposite (take miniature-size photos, as I did Dagron).
What is understood by photomicrograph, ultimately, depends on the context. It may consist of reduce the image of a large object or, on the contrary, enlarge the image of a microscopic object.