Like, sure, they look pretty BUT ARE EVERYTHING BUT USEFUL!
They are exactly the opposite of that!
Human brains are made to recognize colours shapes and paterns, and just having a EXTREMELY SMALL PORTION of an image just destroys everything about that, you can hardly know if an image is what you are looking for or something that you might like just by freaking looking at the face of 1 character on a big composition, or 2 fractions of 2 faces, or a paw , or something in the background.
Looking good doesn't mean functional, and this sites thumbnails are the opposite of functional.
EDIT: I mean, the browser itself is well made but, even if a small pixelated thumbnail of a big composition is well, really pixelated and small, the brain can still figure out the main poses and proportions and lighting and stuff of the being involved in it, and can figure out what it's there, but the artsy thumbnails just break that.