In the Arthur Conan Doyle story "The Adventure of the Dancing Men", the narrator (Watson) shows the reader a hand-drawing of hieroglyphic-like dancing men.

There are illustrations in several places in that story. Can an audiobook convey them? Probably not.

• If you have a used an illustration in your book (a diagram, a map, etc), try to write out a description of it sufficient to provide the listener with a word-picture substitute.
• If the illustration is essential, include it in the package with the recordings.
• Downloadable images are fine, but remember that you are committing to provide valid links. Will the links still work five years from now? Put the burden of maintaining those links on the publisher, not your personal website.
• If there are too many illustrations, consider doing an abridged version audiobook and avoid sections that rely on illustrations.