Two possible options:
- If they are images (jpgs, pngs, ...), as chrisridd said use Image Briwser by double tap. Buuuut, it only works in kepubs. So if your books are epubs you need to use jgoguen's Calibre KoboTouchExtended driver for automatic conversion on the fly.
- Edit the e-book (with Sigil or with the new Edit book in Calibre) and tweak the CSS style of the images to make them bigger.
(For both options you will need DRM free epubs or neither jgoguen's driver nor the editing tools will be able to access the book. An Apprentice who is called Alf, google for it, would be your solution)..