For centuries, skulls both animal and human have been portrayed in art. In Western art, they usually appear as memento morii, reminders that life is short and precious. Other cultures regarded the skull in a different light - for example, the Aztecs portrayed skulls quite frequently in their art, traces of which can be seen in today's Cinquo de Mayo celebrations. In more recent times, artists like Georgia O'Keefe have begun to use skulls in art more for the clean, stark aestheticism of their lines than for the symbolic weight that they carry.
Here, for your delectation, are some fine sites on skull-related imagery and artists who have taken the skull for a theme.