(In the US) Any image you take is copyrighted without any additional action on your part, you fully own this copyright and should NEVER part with the copyright. I would suggest you register the copyright, visit http://www.copyright.gov/ for more info on that. You can file a lawsuit against anyone who violates your copyright regardless of if you register, but registering allows additional (punitive) damages to be collected up to $150,000 per image.

Licensing is something you give others, to reprint, sale on your behalf, display publicly, etc. You should never give up your copyright to an image, read any contract carefully to ensure you don't. If you are going to be printing and selling the cards yourself (even if selling them wholesale, you don't need to license anything. If however you decided to sell them image itself to a card company, you would 'license' it to them, giving them permission to reprint and sell the image. There are lots of example licenses floating around the internet, and if you have any questions on a contract, find a good contract lawyer who has experience in the photo industry. (any pro photog in your area should be able to refer you to someone)

good luck.