When I try to use the sprite in the game, it shows America walking around with a black background. How do I make it transparent?
If you are using this sprite, without modifications, and with the correct version of the program (RPG Maker VX, or RPG Maker VX Ace) there should be absolutely no problem, unless maybe if you renamed the file. Make sure the name of the file is similar to the other "medium size" events sprites.
Otherwise, there can be many reasons this is happening. Google the name of your RPG Maker version and "how to make characters graphics work" or something like that ^^ There are many forums out there, and usually, they have already solved this kind of question a dozen times! ^^