Where can you find a picture of the earth that can be used on a poster that will be resold?
Contact NASA, or go to the NASA websites for information on licensing for their photographic archives. The link below has many photos, and the site claims that all photos are in the public domain. WikiAnswers is not responsible for the contents of the site or any claims made on it. Whatever photo you use, you should make some final checks to be sure that selling the image will not put you at risk in the future. Often, restrictions to the use even of public domain photographs involve the need for a sign-off from anyone who is recognizable in a photo.