They can be viewed in Acrobat, and opened and edited in Photoshop. You'll find them by following the path below. You'll need left, middle, and right.pdf. These three files are 8x8 pixels, and can be a solid color or pattern. Below you can see which sections of the dock are represented by each pdf.
Larger images can produce a spectacular effect.
See this dock image by Graham. Be sure to always work on copies of the original files. To get you started, download this set of dock pdf's contributed by Graham Brown. Unfortunately, you can't save a transparent image as a pdf in Photoshop, so I tricked it by selecting the PDF Print Driver in the Chooser, and printed the tiny image to pdf. Under Page Setup, be sure to turn off any pdf image compression that will make the image milky instead of transparent.