While it's not Answers.com policy to say "use the internet", that's the best approach in this case. You can use any major search engine, or look at the "Large Denominations" page at moneyfactory (dot) gov, the Bureau of Engraving and Printing website.
$500 bills have never been formally withdrawn but it's essentially impossible to find them in circulation today. No bills larger than $100 have been printed since the 1940s and banks haven't been allowed to distribute them since 1969.
Other high denomination bills were $1000, $5000, and $10,000. They too were discontinued in the 1940s and distribution ended at the same time as $500 bills.
There is no current US $500 bill. The last ones were printed in 1945 but carried a 1934 series date. Those bills had a picture of President William McKinley.
Yes, there are many images online. Please see the Related Link.
The $500, long out of print, has an image of President William McKinley.
Andrew Jackson's picture is on the US twenty dollar bill.
On the American $500 bill, William McKinley is featured.
A portrait of William McKinley, the 25th President of the United States, appears on the front of the US 500 Dollar bill.
President William McKinley
The US $50 bill shows a picture of President Ulysses S. Grant.
The White House is on the twenty dollar bill. ($20)
Benjamin Franklin is on the US $50 bill.
Nobody is on the back of the $500 US Dollar note, it just says 'The United States of America' and '500 Dollars'.
William McKinley was on the $500 bill for 1928, 1934, and 1934A