Thumbnail image uses as hyperlink to full-sized image. This gives you the best of both worlds: Visitors can see a preview of the image that is very small in size (53KB) but then view the full images that is somewhat large (3.5MB).
Full-sized image resized using HTML width attribute. This is a bad practice because the 3.5MB full-sized images is downloaded and then resized by the browser. So, you download the big image, but then don't get to see the full quality.
Full-sized image resized using HTML width and height attribute. This is an example of a bad practice because I'm changing the original aspect ratio so the picture looks squeezed.