automatic post thumbnail move on every second post?

Im trying to implement a design where every second post has the post image on top in stead of bottom. Ive created a function that outputs a css class to every second post. Thinking that i would fix it with css i realized that i had NO idea how to float an image up or down inside a div? Can it be done with CSS? And if not, what would you do?

All help is appreciated! ??