I crop my thumbnails, for most of the reasons people have posted above, and I couldn't care less if doing that causes people to not look at my images, as I don't tend to click on full image thumbnails that much (I've already basically seen the entire image, unless it's highly detailed there's no reason to view the larger version).

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If you're joking that's just cruel, but if you're being sarcastic, that's even worse.