You can take a screenshot on your NOOK Color. It takes a bit of doing, but it works. Here’s how:
Press the Home (N) button and the Volume Down (vol-) button at the same time. A camera icon appears on the status bar. Tap it to view the shot in the screenshot gallery. You’ll find them stored on your NOOK in the My Stuff area of your Library. You can crop the shot here and make it your wallpaper, but to move the image to your desktop, you’ll have to connect your Nook via USB, then copy the image from the Screenshots folder to a folder on your desktop.
My guess is the same steps will work for the NOOK Tablet.