The worst that will happen is the copyright holder will contact you and ask that you stop using it. You might even get a cease and desist letter, which will make a nice souvenir of your web adventure. It is valuable to note that all NASA images are public domain, but they do ask that you identify NASA as the source of the image. Keep the size small. This is especially important if you create your own graphics.
Images files can be very large, frequently into the megabytes.
This creates a problem in that it will take a long time for the web browser to display the image, especially if you have a slower internet connection. Try to keep file sizes under 100 Kb. Under 50 Kb is even better, and less than 10kb is best. Macromedia Fireworks and Adobe Photoshop can both reduce image sizes for web purposes. Your web page needs to go on a server.