Could the director of the US Mint decide unilaterally that, from tomorrow going forward, all US coins bear the image of Marilyn Monroe on the obverse and Spongebob Squarepants on the reverse? Do changes to US coinage/currency require some sort of congressional and/or executive approval?
Everyone has bosses and while I am unsure who can exert control over the director of the US Mint I am sure someone could stop this from happening.
There is a slideshow on the website of the US Mint here, slide #9, that says
, and references the Dollar Coin Act of 1997 wiki link, which led to the Sacagawea Coin.
Here’s another write up on answer.com.
They don’t post cites, though. 
NPR recently did a storyon the Congressional mandate to produce the Presidential and Sacagawea dollar coins, nevermind the fact that most of them are winding up being returned by banks to be stored in Federal Reserve vaults. Best bit is the Congresscritter who fought for the quota of Sacagawea coins (in addition to the Presidentials) still won’t admit that it might not have been the best idea…