Browse sites such as WebMD for images of and information about a variety of anatomical structures, including the bowel. Offline resources include libraries and bookstores, which often have anatomy textbooks or medical magazines with the desired images.
There are many sites on which a visitor can interactively examine human intestines (and other anatomy as well), such as innerbody.com or biodigital.com. For more detailed or specialized images of human bowels, purchase an anatomy and physiology textbook, such as the famous anatomy text "Gray's Anatomy" by Henry Gray, which also includes information on the bowels along with pictures.
For more image-oriented text, an anatomy atlas may be preferable. Additionally, many medical textbooks contain pictures of the human bowel. Medical textbooks may also illustrate differences between healthy intestines and intestines that are affected with some sort of illness. These may be found in book stores or libraries or purchased online on sites such as Amazon.com.