image can refer to 2D renditions in pixels of text (scans, rendering), as well as photos and drawings enclosed in an ebook.
Images can occur in ebooks as extensions of the text illustrating a particular topic (JPEG in pdf, PNG and JPEG in epub).
Images can also be the only content of ebook, in that case the ebook files are often, but not always, composed images of scans of real books. Most common file formats for these image based ebooks are djvu and pdf.
