They can range from little artsy accents on the page numbers, all the way up to full-blown illustrations. I personally love all incarnations of art in written works, and believe they can enhance the experience for the reader. Here are a few examples from Honey Queen, by Christina Mercer:
You'll see there's a bee hovering at the page number in the bottom right corner, and here's a closeup of the chapter headings:
Christina includes titles for each of her chapters, and I think this sort of artwork is a great way to draw the reader's attention to each one, while also tying it in with previous chapters.
For Crow's Rest, I had already bought a bunch of vector artwork of crow silhouettes and wanted to sprinkle them throughout--fortunately, Errick A. Nunnally, who designed the interior layout, went along with it and I love how it turned out:
|The title page|
|The top of each page corner features a tiny crow, and what else do you use for section breaks but a line of crow tracks?!?|
|These are the chapter numberings|