The Internet makes it incredibly simple to beef up a presentation with images on just about any subject. There are many stock photo websites offering images, illustrations and vector-based art for a price. Other sites display "free" photos for anyone to use, with or without restrictions. Here are three types of copyright restriction every presenter should know.

Public Domain: Artwork placed in the Public Domain simply means the person who created that image has decided not to enforce any copyright protection. It could be an individual who just wants to share the work with others, or it could be artwork created by a government body with public funding. An example of the latter would be the images of Earth taken by NASA astronauts. Because the funding for space exploration came from the Federal Government, NASA releases their images for public use.

An exception to Public Domain "freedom" is that an image featuring people or products still have limitations attached. A photo of a Coca-Cola bottle may find itself into a Public Domain collection, but the shape of that bottle remains a trademark of the Coca-Cola Company. Likewise, unless a model release is on file with the photographer, images with identifiable people (clear or close-up faces) should be used with care. Imagine how you would feel if your own image appeared in a presentation about foot fungus or bad breath!

There is a big difference between something being public, and Public Domain. It is not legal to use a photograph from any website just because it is viewable by "the public." Even a picture of Granny's 80th birthday has the same copyright as the works of Ansel Adams or Dorothea Lange. If you find an image from an individual's homepage using a search engine, it may be as simple as asking permission to use the file in your show. You might be surprised how many people would be flattered they had taken a useful shot and give you the go ahead. Especially for a low profile presentation in a classroom or a small business meeting.

Royalty-Free: RF images are generally released for use in any project presentations, printed works, and multimedia and for extended periods of time. They are often priced by the size of the image, with larger images costing more than smaller ones. Restrictions may still apply, especially when it comes to how the images will be distributed to third parties.

Many websites offering Royalty-Free images combine the portfolios of numerous artists to create a larger database of photographs. These artists agree to the Royalty-Free terms in exchange for compensation, making the website a sort of middleman to the PowerPoint user. The copyright for an image is retained by the photographer, and is "loaned" to the end user for their presentation.

Rights-Managed or Rights-Restricted: Managed artwork pricing is based on a variety of factors; length of use; the delivery method; how many people will see it (impressions); who will see it (public or private), etc.

A photo shown in PowerPoint during a small town church sermon would cost significantly less than a photo used in a presentation during the press conference of a new automobile.

As with Royalty-Free images, the copyright is still held by the photographer in most cases. A photographer shooting a specific assignment for a client, or on the payroll of a stock company may turn over those rights based on predetermined agreements with the end-user or agency involved.

Large and notable websites like Corbis and Getty Images built up their businesses on Rights Managed photography. Both now offer Royalty-Free options as well. Many of the smaller Royalty-Free websites, known as "micro-stocks," thrive on low prices and high volume.

When using any image in a PowerPoint presentation, it is important to understand the restrictions involved. When in doubt, talk it out. Contact the person or agency offering the photos and know your rights!