Using satellite and other aerial imagery as a transparency over your plot allows you to accurately trace fairways, greens, tees, water, etc. and place objects in the correct location on your plot.
Generally a higher resolution image makes it easier to layout your course. This tutorial explains how to get the highest resolution image from your source. It's not the only factor though. Shadows, the season an image was taken in, how old an image is, etc. can make different parts of your image difficult to use or inaccurate. Because of this it is better to use multiple images if they are available for your location.
Below is a list of sources you can download imagery from:
This tutorial goes into detail on downloading from the following sources:
Back to Terrain and Imagery Tutorial Main Page