The human skull is a powerful image - it instantly reminds us of our mortality and evokes that subtle fear abiding in all of us of what lies beyond. It should come as no surprise that this symbol is routinely used in the horror genre. After all, horror fiction is all about fear, and what is a more compelling than the fear of death, epitomized by the very face of death - the skull.
Of course, being a default image has its problems. A lazy or unimaginative illustrator can always fall back on the skull. Indeed, there are legions of lackluster skulls adorning thousands of forgettable paperbacks and movie posters. Were I to spend the next year compiling them for you, I wouldn't come close to scratching the surface. As with many of these 'artful conception' posts, I've put together mosaics of examples, both good and bad. However, these don't even begin to scratch the tip of the iceberg. Hopefully, you'll enjoy this small sampling nonetheless.