If you are talking about a picture in a book, if the publisher formatted the book with the zoom function, the picture acts like a footnote. You click on it and it takes you to a page with only the picture or illustration or map or chart on it that fills the whole page. We're still not talking about great resolution here...simple line drawings are fine; maps of NYC will still be illegible.
If you're just asking about making the words bigger...the font size key is on the bottom right of the keyboard.