There is probably hardly any sites in the internet that don't use some thumnails somewhere on their pages. If they have a gallery especially, you can guarantee that they have thumbnails for they are the base starting point in any internet-based gallery of images.
Thumbnail sales is big business, and, along with the sale of full-sized images, have created a huge online business turning over millions of dollars every year in revenue. Of course, buying thumbnails is not always necessary. As long as you have a bigger image to start with, then making it smaller, both in height and width as well as in resoltion is all there is to making a thumbnail.
The use of thumbnails, as previously stated, is mostly for online galleries. However, their use is not limited to just that. They also come in handy when making link buttons for websites, or montage banners.
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Floating Thumbnails. Their site can be found at http://www.realworldstyle.com/thumb_float.html.