Interestingly, "Lord of the Flies" was the not the original title William Golding wanted for this novel. He first titled it something much blander. But typically, others - including publishers - wanted something more vivid and attention grabbing. In England, the graphic title certainly did it no harm as word began to spread about it and more and more people wanted to buy the book.
As to symbols that the "Lord of the Flies" may represent, here are some essay pointers and themes you may want to revisit - make a brief list and ask yourself which of the novel's themes best fit the ghastly image.
Order/Disorder - Good/Evil - Death/Life - Decay - Anarchy - Free Will - Convention/Freedom - The Fight of Angels and Demons in Heaven/Hell - Falling From Grace - Hunting - Trophy - Humiliation - Sacrifice - Ritual Exhibition
coachingcorner provides a great list of themes to explore re. symbolism in Lord of the Flies (i.e., Order/Disorder - Good/Evil - Death/Life - Decay - Anarchy - Free Will - Convention/Freedom - The Fight of Angels and Demons in Heaven/Hell - Falling From Grace - Hunting - Trophy - Humiliation - Sacrifice - Ritual Exhibition).
There is a biblical allusion in the book's title that may be of help in your exploration of these themes. Interestingly, "Lord of the flies" is a synonym for "Beelzebub" (a.k.a. the devil).
Here's the derivation breakdown: ORIGIN from late Latin Beëlzebub, translating Hebrew ba‛al zĕ b ū b ‘lord of flies,’ the name of a Philistine god (2 Kings 1:2), and Greek Beelzeboul ‘the Devil’ (Matt. 12:24). Source: Any good (print/electronic) dictionary.
As you read coachingcorner's list of themes again, this tidbit will surely/hopefully help you with an essay, test, or simply to gain greater insight into the novel.
Hope this helps. :)