Luckily you can do this without killing anyone! If you buy the PDF, you can open it in Adobe Acrobat Reader, use the Selection Tool to highlight the image you like, click the "copy" button, open your favorite image editor (such as GIMP), paste, crop/scale the image for your desired screen resolution, and voila! You've got your wallpaper. If any images don't copy correctly using that method, you can always use your operating system's screenshot feature.