Thumbnails are smaller, lower resolution versions of larger images. The purpose of making thumbnails is to allow your web site visitors to choose which images they'd like to wait for and allow them to skip images they do not want to spend time to download. Never create a page full of large images.
Thumbnails are easy to make.
In Photoshop , open up your full size image. In this example I am working in pixels. This will make your thumbnails much easier to browse. The actual URL for the image is not really a HTML document, it's just the name of the graphic. Then the visitor will be required to hit the back button to get back to your thumbnail page.