Except of Terragen renders, all images in my Rendo gallery have thumbnails being cropped part of the corresponding image, a teaser of some sort. My take on this is that if you already seen whole image as sized-down thumb, what's the point of clicking it to see it once again, only bigger ? I'm a very curious creature myself, and I do prefer teasing thumbs - I rarely click thumbs which seem to be whole image, as it rarely makes me interested. I rather click nicely rendered eye to see if rest of image is as nice too, than badly sized-down pic to check if it really looks better when bigger (and believe me - too often it doesn't).
Some posts in this thread seem to be about "people don't make nude thumbs anymore so I can't see if there are any T&A or not, so don't know if the pic is worth viewing", which is itself interesting ;)

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Outdated gallery over at DeviantArt

Fics at FanFiction.net and Archive of Our Own (AO3)