The fz_image structure is used to represent a generic Image object in MuPDF. It can be viewed as an encapsulation from which both a rendering of an image (as a fz_pixmap) and (often) the original source data can be retrieved.
Further discussion of fz_images is deferred to chapter 24 Image Internals in Part 2. While it is perfectly permissible for a caller to create images, in most cases they will treat them as ‘black boxes’ to just be passed around.