Below is a thumbnail image, which is inside a hyperlink. The hyperlink is to a bigger image.
Thumbnails are useful because they make the page load faster. The thumbnail above has a filesize of 41.2 kB. The image it links to has a filesize of 1737 kB, not to mention being almost ten times wider and taller. It's much faster to download the thumbnail, and that's what counts towards the time to load the page itself.
Please, do inspect this page with right click → view source.