The term 'format' is confusingly applied to both the content (e.g. an image) and the carrier medium (e.g. a painting, or a photograph). It is important to distinguish between the type of content and the carriers (physical and digital media) that are used to convey the content.
In the past libraries tended to just hold information as text, with perhaps a small amount of images in the form of maps, but today the range of materials is wider.
Some carriers hold only one type of content, but others can hold multiple content types. A reel of film carries only projected images, whereas a piece of paper may carry text, notated music or an image.
Digital files (of text, images, datasets, etc)