How did they decide who's faces go on what bills?

Congress decides what the money looks like. It works like any other form of legislation.

It's interesting that many most early Americans, including Washington and Jefferson, were adamantly opposed to having the likeness of a real person on American money. It was argued that it was too much like royalty (who often had their image on coins). All US coins had a Miss Liberty instead of the likeness of a real person until the Lincoln penny. Poor Washington and Jefferson are on both coins and bills!

I beleive the talking portraits that hang in the Oval Officer are consulted.

I think the Fed does it for irony.

How else would Andrew "I beat the bank" Jackson have gotten on the twenty dollar federal reserve note?

I'm more curious about the bills on the faces,,,,,

I'm a platypus, damnit, whys everbody thinking I'm a duck

Everytime I see Jackson on a $20.00 I think how mean spirited that was.