A lot of it is starting with a good original. In the link, right now, there's a person shot as a photojournalistic portrait. The man himself is in sharp focus, but the background is thrown out of focus. This is an optical phenomenon called depth of field or bokeh. The contrast between the sharp focus and soft background makes the subject appear sharper than they really are.
Another consideration is that when you resample an image, even when you shrink it, the software applies a Gaussian blur first. It's part of the algorithm; you can read more here. So ... after downsizing an image, you want to sharpen it a little. I find 0.2 pixels radius, 80 % or so amount and 0 threshold works as a good starting point for thumbs.

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