Hey,
A) depends if you want to hardcode it into your theme, eg:
- create a folder, 'banners' or like
- create a function, inside functions.php which reads all *.jpg into an array, then use shuffle() to mix em up, and print the first element
B) or the ability to add banners from Admin area,
- you'd have to create a form on your admin so you can load the files into a folder like above^, then the process is pretty much the same.
C) There might be a plugin already that gets you there or halfway,i dont remember one right now