There’s not really a concept of a category for photos. Instead, by default, the photos page lists only JPEG images, the rationale being that a .jpg is probably a photo while .gifs and .pngs are most likely screenshots or illustrations. Some themes and plug-ins might change this behavior, though.
So, unless you want to get your hands dirty with some custom coding, the way to go is to make sure that all photos are .jpgs (and that none of your screenshots and memes are).
Photo pages document some technical details.