The thumbnail is the first impression a viewer gets of a piece of artwork. There's a great deal of thought and effort that goes into creating a good composition at a distance, and this effort is usually ruined by cropping the edges to create a square thumbnail.
Additionally character shots end up with heads cropped off, and the focal point getting shifted.
This is bad for both artists and people who are seeking art, as they do not get to see the piece in it's intended form.
While this may make the webpage look cleaner, it makes the artwork contained look worse.
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