[un1versal]

That's not simple at all, an image that expresses that visually in a universal iconic recognizable manner that is simple to understand by all in a non textual manner, is hard to achieve in very least.

What would be conceivably easier, since you cant blur the actual image is to overlay a blurred solid color image over the top and have its opacity to 70% or 80% so you can see the original thumb underneath but not so much as you could easily glamour the possible visual spoiler. Something for thumbs like the dim screen-saver type idea.