Do I need to trademark each individual postcard design I create for my stationery business?

You should not use the "R" with the circle around it unless the mark is federally registered. Also, don't confuse trademark registration with copyright registration. An artistic design might better be protected via copyright registration, depending on how it is being used.

James is right on the money. While it is possible to trademark a particular design that identifies a product or service, most artistic works are more suitable for copyright protection. One common example of a design that is trademarked is Nike's "swoosh" - the infamous curved check-mark that identifies Nike's sneakers, apparel and sporting goods.

On the other hand, artistic works such as paintings, drawings, or even postcards are better suited for copyright protection. In order to have copyright protection over your work, you simply add the letter c enclosed in a circle, followed by the year the work was created and either a name or abbreviation to identify your design. For example, "(c) 2009 Nike Corp." might identify a copyrighted work created by the Nike Corporation. Indeed, you are not even required to register your work with the United States Copyright Office if you so desire (although registering your work affords greater protection and valuable legal benefits should someone infringe on your copyright).

James is further correct that you should not use the letter R enclosed in a circle if you do not have a federally registered trademark. The (R) symbol is reserved solely for trademarks that are registered with the United States Patent and Trademark Office (USPTO). On the other hand, you may use the symbol TM to put the world on notice that you are claiming common law trademark protection over your work. If you chose to trademark your design for postcards and other stationary products, you may use the TM symbol next to your design even while your application with the USPTO is pending.

I am not your lawyer.

The original works of art that you are putting on your postcards are works of authorship and hence should be copyrighted and not trademarked. They do not show a source of goods or services. Rather they are art. If you start using the art to denote your goods (you sell physical reproductions of the artwork in 3 demensions for example) then you can proceed with a trademark as you will have a good associated with the artwork design.

Designs may be trademarked. But only if they are logos or products themselves (think about the Coca Cola bottle that has spirals).

Start with the copyright filing with the Library of Congress. At the same time file an Intent-to-Use Application with the US PTO for the design that you plan on the stationary products.

Consult with a knowledge IP/Copyright/trademark lawyer or you'll prejudice your rights.

Good luck.