A thumbnail image is a smaller version of the image itself. The thumbnail is used as a link that when clicked, will load the full-sized image. It contains all of the most important aspects of an image, but is considerably smaller.
For ease of placement on the screen, images in a group are usually reduced to approximately the same thumbnail size as each other. In other words, the thumbnail for a large image is roughly the same size as the thumbnail for a medium-sized image.
Loading image can take long time, depending on the size and the complexity of the image. Using a thumbnail image gives a visitor the opportunity to decide whether to view the full-sized image.
Creating a thumbnail image does not modify the original image; it instead generates a new, but smaller, image.
The image is typically reduced to a predetermined maximum dimension while constraining its proportions so as not to distort it. This maximum dimension is usually applied to the width or the height, depending upon whether the image’s orientation is landscape or portrait.