Are you looking to rotate through them per page load (so that would be server-side like PHP) or client-side (which would be JavaScript's job) -- not really something you could do with just HTML and CSS... Well, CSS3 would help a lot, though a lot of animations for switching between them like opacity doesn't work on iframes...
.. and if you don't need them animated, JS could do it really simply with one iframe just by swapping it's 'src'.
Need more on how you want to 'loop' them before a real solution can be given.
