"Your work is under copyright protection the moment it is created and fixed in a tangible form that it is perceptible either directly or with the aid of a machine or device."
http://www.copyright.gov/help/faq/faq-general.html#mywork

Unless you specify otherwise or there is something in the terms for the website/contest/whatever it is assumed you retain full copyright of your work. On Flickr they give you the option of changing the copyright license from all rights reserved to something else where you give other people the rights to do certain things with your image.

I'm not sure about proving that you are the original author or how you would go about proving that.