There are many different types of photo galleries available.  Most allow for easy adding of images to the slideshow, either by selecting from images on one's workstation or the server, pointing the gallery module at a particular file folder, or by selecting from other tables/structures within a site. Each gallery can be customized in many ways. 

Here are examples of some of the modules available:

• Smooth Gallery
• Popeye

Here are a few examples examples of galleries on actual sites:

• Osvaldo Reyes, artist (custom gallery with lightbox)
• Spinning Wheel Design Home Page (flash slideshow)
• Spinning Wheel Design Portfolio Page (slideshow)
• AVW Telav (slideshow with thumbnails)