Do I retain copyright of my pictures?
People can purchase your picture, based on the price you set for it. If someone does, you have granted permission for them to use your picture in a brochure, website, personal print, or other one time use. They may not resell your picture.
You still retain full copyright, even after a picture is purchased.
If you do not wish your photograph to be used, simply set your price at a very high level, say $3,000,000. There is no limit to how high of a price you can set your picture.