Ancestry displays the census and BMD images as lots of images. For example, for a BMD index page they may have 2 images wide by 4 images high. If you open an image editing program, go back onto the BMD image, press print screen, and then paste the image onto a new file in the image program, you will have a screenshot of your desktop. Then using the tools your image editing program has, you can crop the image, and then save it.
Andrew