The URL in the clue is a hot link to a TwitPic, a detail of Michelangelo's utterly breathtaking work on the ceiling of the Sistine Chapel at the Vatican, specifically a detail of the face of God in a section dealing with the creation of the Sun and the Moon (shown above).
However, in the TwitPic, the face of God is overlayed with five numbers in Roman numerals. John Stump reported the solution to this code on Dan Brown's Facebook fan page, within a few minutes of the clue's posting. (Congratulations, John Stump!)
The five Roman numerals represent the 4th, 5th, 9th, 19th, and 13th letters of the English alphabet, thus spelling out the word DEISM.
[I quote below, with some modifications, from one of my other blogs, "Freemasonry: Reality, Myth, and Legend," where I recently considered the matter of Deism in a post at some length.]
Theism vs. Deism
There are many ways to conceive of God. One set of ways comes under the heading of Theism, a group of ideas about God that encompasses Judaism, Christianity, and Islam.
A theistic notion of God takes the position that God created the universe and is vitally concerned with its affairs, including the affairs of humankind; God watches over us, individually and collectively (part of the meaning of the 'Eye of God' symbol, incidentally). A theistic God communicates the Divine Will to people from time to time, for their benefit -- the process of revelation, which may result in the accumulation of scriptures as records of revelation. A theistic God may work miracles to serve divine purposes. Those who believe in a theistic notion of God are called theists.
A different way to conceive of God is the position of Deism. In the deistic perspective, God created the universe, but does not particularly intervene in the affairs of the universe or humankind. To some extent, humankind may come to know a deistic God through the workings of human reason, but a deistic God does not communicate through revelation; thus, whatever people designate as 'scriptures' are merely human creations and interpretations. A deistic God does not interfere with the workings of natural law, and so does not work miracles. A deistic God gave reason and compassion to all of humankind, but otherwise does not actively intervene in the lives of individuals. Those who believe in a deistic notion of God are called deists.
The Founding Fathers and Deism
Some prominent American founding fathers were deists. Certainly this is so for George Washington, Benjamin Franklin, Thomas Jefferson, and James Monroe; the case for their being deists is made convincingly by David L. Holmes, in his brief, excellent, and accessible book, The Faiths of the Founding Fathers (New York: Oxford University Press, 2006). (Of course, the French philosopher Voltaire was a noted deist. Voltaire was a major intellectual influence on the Founding Fathers, so much so that I have nominated him as the answer to Clue #29 as the French 'honorary founding father.')
Relevance to The Lost Symbol
It is clear that the U.S. Founding Fathers will play a major role in this novel in some way. As I have mentioned, several of them were deists. Dan Brown likes to bring up different positions on matters of faith, religion, and spirituality; no doubt he will address deism as a potential position, perhaps one agreeable to his hero, Robert Langdon.
One thing I hope Dan Brown does not do is perpetuate the oft-repeated falsehood that Freemasonry is inherently deist. It is not; if anything, Freemasonry is skewed in the direction of theism, a case I make here.
In any case, it will be a welcome experience to see religion addressed seriously in popular literature.